A tritrophic relationship involving the plant Croton floribundus Spreng (Euphorbiaceae), an unidentified gall-midge belonging to either Clinodiplosis Kieffer 1895 or Alycaulus Rübsaamen 1916 (Diptera: Cecidomyiidae), and a new species of parasitic wasp belonging to Omphale Haliday 1833 (Hymenoptera: Eulophidae) is described. The gall and Omphale japii sp. nov., are described and illustrated. This is the third record of Omphale from Brazil and the new species is compared to other Omphale species from the Neotropical region.1333437Bouček, Z., Askew, R.R., (1968) Index of Palaearctic Eulophidae, 3, p. 254. , (excl, Tetrastichinae). Le Francois, Paris, Index of Entomophagous InsectsCorrea, M.P., (1984) Dicionário das plantas úteis do Brasil e das exóticas cultivadas, 1, pp. 503-504. , Imprensa Nacional, Rio de JaneiroDe Santis, L., (1979) Catálogo de los himenópteros calcidoideos de América al sur de los Estados Unidos, p. 488. , Publicacion Especial, La PlataDziurzynski, A., The inhabitants of the galls of Mikiola fagi Htg. Part I. Materials for the morphology and development of Mikiola fagi Htg. (Itoniidae), as well as of its endophagous primary parasite Secodes coactus Ratzb. (Chalcididae) (1961) Acta Zool. Cracov., 6, pp. 9-49Fernandes, L.B.R., Dias Filho, M.M., Fernandes, M.A., Penteado-Dias, A.M., Ichneumonidae (Hymenoptera) parasitoids of Lepidoptera caterpillars feeding on Croton floribundus Spreng (Euphorbiaceae) (2010) Rev. Bras. Entomol., 54 (2), pp. 263-269. , http://dx.doi.org/10.1590/S0085-56262010000200009Gibson, G.A.P., Morphology and terminology (1997) In Annotated Keys to the Genera of Nearctic Chalcidoidea, pp. 16-44. , (Hymenoptera) (Huber, J.T., Woolley J.B.). NRC Research Press, Ottawa, 794pGraham, M.W.R.V., Keys to the British genera and species of Elachertinae, Eulophinae and Euderinae (Hym., Chalcidoidea) (1959) Trans. Soc. Brit. Entomol., 13, pp. 169-204Graham, M.W.R.V., Additions and corrections to the British list of Eulophidae (Hym., Chalcidoidea), with descriptions of some new species (1963) Trans. Soc. Brit. Entomol., 15, pp. 167-275Hanson, P.E., Gauld, I.D., Hymenoptera de la Región Neotropical (2006) Mem. Am. Entomol. Inst., 77, pp. 1-994. , (eds.)Hansson, C., Taxonomic revision of the Nearctic species of Omphale Haliday (Hymenoptera: Eulophidae) (1996) Entomol. Scand. Suppl., 49, pp. 1-78Hansson, C., Mexican species of the genus Omphale Haliday (Hymenoptera: Eulophidae), a taxonomic study (1997) J. Hymenoptera Res., 6, pp. 107-151Hansson, C., Eulophidae of Costa Rica, 2 (2004) Mem. Am. Entomol. Inst., 75, pp. 1-537Lasalle, J., Schauff, M.E., Preliminary studies on Neotropical Eulophidae (Hymenoptera: Chalcidoidea): Ashmead, Cameron, Howard and Walker species (1992) Contrib. Am. Entomol. Inst., 27, pp. 1-47Maia, V.C., Fernandes, G.W., Insect galls from Serra de São José (Tiradentes, MG, Brazil) (2004) Braz. J. Biol., 64 (3), pp. 423-445. , http://dx.doi.org/10.1590/S1519-69842004000300007Medina, J.M., Peixoto, J.L.B., Silva, A.A., Haraguchi, S.K., Falavigna, D.L.M., Zamuner, M.L.M., Sarragiotto, M.H., Vidotti, G.J., Evaluation of the molluscicidal and Schistosoma mansoni cercariae activity of Croton floribundus extracts and kaurenoic acid (2009) Rev. Bras. Farmacogn., 19 (1 B), pp. 207-211. , http://dx.doi.org/10.1590/S0102-695X2009000200005Noyes, J.S., Collecting and preserving chalcid wasps (Hymenoptera: Chalcidoidea) (1982) J. Nat. Hist., 16, pp. 315-334. , http://dx.doi.org/10.1080/00222938200770261Noyes, J.S., (2012) Universal Chalcidoid Database, , http://www.nhm.ac.uk/research-curation/research/projects/chalcidoids/database/, (último acesso em 04/04/2012)Price, P.W., Bouton, C.E., Gross, P., McPheron, B.A., Thompson, J.N., Weis, A.E., Interactions among three trophic levels: Influence of plant on interactions between insect herbivores and natural enemies (1980) Ann. Rev. Ecol. Syst., 11, pp. 41-65. , http://dx.doi.org/10.1146/annurev.es.11.110180.000353