This research aims to study the effects of the traditional management of Tabebuia cassinoides (Bignoneaceae) on the intrapopulation genetic variability and the mating system of a managed population at the Iguape, SP and a natural population at the Ecological Station of Jureia, Iguape, SP. The sampling in natural and managed populations included 11 and 10 maternal families with 10 individuals per family. The mean fixation index within populations (F̂ IS) and the fixation index over all populations (F̂ IT), were positive, high and significant (0.259 and 0.282, respectively), suggesting, a relatively high inbreeding in the populations. The genetic divergence between populations (F̂ ST) was low for the mean over loci (0.031), revealing that approximately 97% of the genetic variability was distributed within populations. The natural population had alleles that were absent in the managed population. For the natural and managed populations the following estimates were obtained: observed heterozygosity (H o), 0.253 (0.041) and 0.174 (0.036); genetic diversity (Ĥ e), 0.314 (0.051) and 0.266 (0.049); percentage of polymorphic loci (P), 84.6% and 76.9%; fixation index (f), 0.192 (0.069) and 0.346 (0.072); multilocus outcrossing rate (t̂ m), 0.895 (0.048) and 0.783 (0.104); self-pollinization rate (Ŝ), 0.105 and 0.217, respectively. Results indicated that forestry management in this case lead to a loss of rare aleles, a reduction in heterozigosity, genetic diversity and percentage of polymorphic loci, an increase in the fixation index and a tendency towards an increase in the self-pollinization rate in the managed population. Finally, evaluations of the effective population size indicated that for the management of this forest to be genetically sustainable, at least 60 reproductive individuals should be kept per ha. 58127143Alfenas, S.A., (1998) Eletroforese de isoenzimas e proteínas afins: Fundamentos e aplicações em plantas e microrganismos, , Viçosa: UFV, 574pAllard, R.W., (1971) Princípios do melhoramento genético das plantas, , São Paulo: Edgard Blucher, 381pAlvarez-Buylla, E.R., García-Barrios, R., Lara-Moreno, C., Martínez-Ramos, M., Demographic and genetic models in conservation biology: Applications and perspectives for tropical rain forest tree species (1996) Annual review ecology systematics, 27, pp. 387-421Bawa, K.S., Krugman, S.L., Reproductive biology and benetics of tropical trees in relation to conservation and manegement (1990) Rain forest regeneration and management, pp. 119-136. , Gomes-Pompa, A.Whitmore, T. C.Hadley, M. Paris: UNESCOCarvalho, P.E.R., (1994) Espécies florestais brasileiras: Recomendações silviculturais, potencialidades e uso de madeira, , Brasília: EMBRAPA-CNPF, 640pCrow, J.F., Kimura, M.A., (1970) An introduction to population genetics theory, , New York: Harper and Row, 591pEllstrand, N., Elam, D.R., Population genetic consequences of small population size: Implications for plant conservation (1993) Annual review on ecological systems, 24, pp. 217-242Falconer, D.S., Mackay, T.F., (1996) Introduction to quantitative genetics, , London: Longman, 464pFowler, D.P., Hybridization of black spruce and Serbian spruce (1980) Information, M-X-112, pp. 1-30Frankel, O.H., Soulé, M.S., (1981) Conservation and evolution, , Cambridge: Cambridge University Press, 327pFutuyma, D.J., (1992) Biologia evolutiva, , Ribeirão Preto: Sociedade Brasileira de Genética, 646pGeburek, T., Some results of inbreeding depression in Serbian spruce (1986) Silvae genetica, 35 (4), pp. 169-172. , Picea omorica (Panc.) PurkHamrick, J.L., Allozyme diversity of natural stands versus seed orchard loblolly pine (1991) Meeting of the Canadian Tree Improvement Association, 23, pp. 1-21. , Ottawa, 1991. Proceedings. OttawaHamrick, J.L., Godt, M.J.W., Allozyme diversity in plant species (1990) Plant population genetics, breeding and genetic resources, pp. 43-63. , Brown, A.H.D.Clegg, M.T.Kahler, A.L.Weir, B.S., ed.. Sunderland: SinauerKephart, S.R., Starch gel electrophoresis of plant isozymes: A comparative analysis of techniques (1990) American journal of botany, 77 (5), pp. 693-712Koelewijn, H.P., Koski, V., Savolainen, O., Magnitude and timing of inbreeding depression in Scots pine (1999) Evolution, 53 (3), pp. 758-768. , Pinus sylvestris LLi, C.C., Horvitz, D.G., Some methods of estimating the inbreeding coefficient (1953) American journal of human genetics, 5, pp. 107-117Loiselle, B.A., Sork, V.L., Nason, J., Graham, C., Spatial genetic structure of a tropical understory shrub (1995) American journal of botany, 82 (11), pp. 1420-1425. , Psychotria officinalis (Rubiaceae)Loveless, M.D., Hamrick, J.L., Distribuición de la variacion en espécies de árboles tropicales (1987) Revista biologia tropicales, 35 (1), pp. 165-175Mettler, L.E., Gregg, T.G., (1973) Genética de populações e evolução, , São Paulo: Polígono / EDUSP, 262pMoraes, M.L.T., (1992) Variabilidade genética por isoenzimas e caracteres quantitativos em duas populações naturals de aroeira Myracrodruon urundeuva F.F. & M. F. Allemão - Anacardiaceae [Syn: Astronium urundeuva (Fr. Allemão)] Engler], , Piracicaba. 139p. Tese (Doutorado) - Escola Superior de Agricultura Luiz de Queiroz. Universidade de São PauloMori, E.S., (1993) Variabilidade genética isoenzimática em uma população de Eucalyptus grandis Hill ex Maiden submetida a diferentes intensidades de seleção, , Piracicaba. 118p. Tese (Doutorado) - Escola Superior de Agricultura Luiz Queiroz. Universidade de São PauloMurawski, D.A., Reproductive biology and genetics of tropical trees from canopy perpective (1995) Forest canopies, pp. 457-493. , Lowman, M.D.Nadkarni, N.M., ed. New York: Academic PressMurawski, D.A., Dayanandan, B., Bawa, K.S., Outcrossing rates of two endemic Shorea species from Sri Lankan tropical rain forest (1994) Biotropica, 26 (1), pp. 23-29Murawski, D.A., Hamrick, J.L., The effect of the density of flowering individuals on the mating systems of nine tropical tree species (1991) Heredity, 67, pp. 167-174Nei, M., F-statistics and analysis of gene diversity in subdivided populations (1977) Annual of human genetics, 41, pp. 225-233Reis, M.S., (1996) Distribuição e dinâmica da variabilidade genética em populações naturals de palmiteiro, p. 210. , Euterpe edulis M. Piracicaba. Tese (Doutorado) - Escola Superior de Agricultura Luiz de Queiroz. Universidade de São PauloRitland, K., (1997) Multilocus mating system program MLTR: Version 1.1, , Toronto: University of Toronto. não publicadoRitland, K., Series of FORTRAN computer programs for estimating plant mating systems (1990) Journal of heredity, 81, pp. 235-237Ritland, K., Jain, S., A model for the estimation of outcrossing rate and gene frequencies using independent loci (1981) Heredity, 47, pp. 35-52Sokal, R.R., Rohlf, F.J., (1981) Biometry: The principles and practice of statistics in biological research. 2.ed., , San Francisco: W.H. Freeman, 859pSoltis, D.E., Soltis, P.S., (1989) Isozymes in plant biology, pp. 46-67. , Portland: Dioscorides PressSẃofford, D.L., Selander, R.B., Biosys-1: A FORTRAN computer program for the analysis of allelic variation in population genetics and biochemical systematics (1989) Journal of heredity, 72, pp. 282-283Wilkox, M.D., Inbreeding depression and genetic variances estimated from self- and cross-pollinated families of Pinus radiata (1983) Silvae genetica, 32, pp. 89-96Workman, P., Niswander, J.L., Population studies on southwestern Indian tribes: 2- Local genetic differentiation in the Papago (1970) American journal human genetic, 22, pp. 24-49Wright, S., The interpretation of population structure by F-statistics with special regard to systems of mating (1965) Evolution, 19, pp. 395-420Wu, H.X., Matheson, A.C., Spancer, D., Inbreeding in Pinus radiata: 1- The effect of inbreeding on growth, survival and variance (1998) Theoretical applied genetics, 97, pp. 1256-1268