Spermicide (i.e., female-mediated sperm death) is an understudied but potentially widespread phenomenon that has important ramifications for the study of sexual conflict, postcopulatory sexual selection, and fertility. Males are predicted to evolve adaptations against spermicide, but few antispermicidal mechanisms have been definitively identified. One such adaptation may be the enigmatic infertile sperm morphs or ‘parasperm’ produced by many species, which have been hypothesized to protect their fertile brother ‘eusperm’ from spermicide. Here, we show that female Drosophila pseudoobscura reproductive tracts are spermicidal and that the survival of eusperm after exposure to the female tract is highest when males produce many parasperm. This study clarifies the adaptive significance of infertile sperm castes, which has remained elusive in Drosophila and other taxa despite much recent interest We suggest that spermicide and male countermeasures against it are more common than is appreciated currently and discuss how spermicide could drive the evolution of several key male traits, including sperm size and number.