Abstract Buschini, M.L.T. and Bergamaschi, A.C.B. 2009. Strongly female-biased sex allocation in a trivoltine population of Trypoxylon (Trypargilum) opacum Brèthes (Hymenoptera, Crabronidae). —Acta Zoologica (Stockholm) 91: 433–439. This study was carried in southern Brazil from December 2001 to December 2004. The aim of this paper is to provide additional information on the life-history and sex allocation of this little known species. This wasp’s species has two alternative life histories: either they pupated immediately and emerged as adults later in the same season (direct development) or they entered diapause, overwintering and pupating the following spring (delayed development). The numerical sex ratio of overwintering and of direct developing wasps were strongly female biased in 2002, 2003 and 2004.