Seven species of Baeus occur in the Neotropical and Andean regions: B. achaearaneus Loiácono, B. auraticeps Girault, B. kuscheli Ogloblin, B. latrodecti Dozier, B. metazygiae Loiácono et Margaría, B. platensis (Bréthes), and B. ventricosus Ogloblin. All of them are associated with spiders of the families Araneidae and Theridiidae. In this study, a new species of Baeus is described from egg sacs of Cyclosa morretes (Araneidae) collected in a natural reserve in São Paulo State, Brazil. The spider species is a new host for Baeus. We also evaluated the frequency of attacks and the impact of parasitism on the spider egg sacs.1172181187Austin, A.D., The fecundity, development and host relationships of Ceratobaeus spp. (Hymenoptera: Scelionidae), parasites of spider eggs (1984) Ecological Entomology, 9, pp. 125-138Austin, A.D., The function of spider egg sacs in relation to parasitoids and predators, with special reference to the Australian fauna (1985) Journal of Natural History, 19, pp. 359-376Austin, A.D., Johnson, N., Dowton, M., Systematics, evolution, and biology of Scelionid and Platygastrid wasps (2005) Annual Review of Entomology, 50, pp. 553-582Van, B.P., Sunderland, K.D., Topping, C.J., Eggsac parasitism of money spiders (Araneae, Linyphiidae) in cereals, with a simple method for estimating percentage parasitism of Erigone spp. eggsacs by Hymenoptera (1994) Journal of Applied Entomology, 118, pp. 217-223Brèthes, J., Himenópteros de la America Meridional (1913) Anales del Museo Nacional de Buenos Aires, 24, pp. 35-165Dozier, H.L., A new scelionid egg parasite of the black widow spider (1931) Proceedings of the Entomological Society of Washington, 33, pp. 27-28Girault, A.A., Notes on two South American parasitic Hymenoptera (1915) The Entomologist, 48, pp. 213-214Gillespie, R.G., Costs and benefits of brood care in the hawaiian happy face spider Theridion grallator (Araneae, Theridiidae) (1990) American Midland Naturalist, 123, pp. 236-243Gonzaga, M.O., (2004) Funções e Variabilidade Estrutural Dos Estabilimentos Construídos Por Cyclosa Fililineata Hingston 1932 e Cyclosa Morretes Levi 1999 (Araneae: Araneidae), , Ph.D. thesis, Universidade Estadual de Campinas, SP, BrazilGonzaga, M.O., Vasconcellos-Neto, J., Testing the functions of detritus stabilimenta in webs of Cyclosa fililineata and Cyclosa morretes (Araneae: Araneidae): Do they attract prey or reduce the risk of predation? (2005) Ethology, 111, pp. 479-491Johnson, N., (2004) Scelionidae, , http://atbi.biosci.ohio-state.edu:210/hymenoptera/eol_scelionidaeLevi, H.W., The Neotropical and Mexican orb weavers of the genera Cyclosa and Allocyclosa (Araneae: Araneidae) (1999) Bulletin of the Museum of Comparative Zoology, 155, pp. 299-379Loiácono, M.S., Notas sobre Proctotrupoideos Neotropicales I (Hymenoptera) (1973) Neotrópica, 19, pp. 138-140Loiácono, M.S., Margaría, C.B., Las especies del género Baeus (Hymenoptera: Scelionidae) endoparasitoides de ootecas de arañas en la región neotropical (2004) Acta Zoológica Mexicana, 20, pp. 83-90Masner, L., Denis, J., The Nearctic species of Idris Foerster. Part I: The melleus-group (Hymenoptera: Scelionidae) (1996) Canadian Entomology, 128, pp. 85-114Morrone, J.J., Biogeografía de América Latina y el Caribe (2001) M&T-Manuales & Tesis SEA, , CYTED, ORCyT-UNESCO & SEA (Eds.), 148 ppOgloblin, A., Los insectos de las Islas Juan Fernández (1957) Revista Chilena Entomología, 5, pp. 413-444Valerio, C.E., A unique case of mutualism (1974) American Naturalist, 109, pp. 235-238Valerio, C.E., Two alternative strategies for spider egg parasitoids (1984) Revista de Biología Tropical, 32, pp. 123-128Vetter, R.S., Bruyea, G.P., Visscher, P.K., The use of bleach to dissolve spider silk (1996) Bulletin of British Arachnological Society, 10, pp. 146-148