Abstract The microsporidium Nosema muscidifuracis was found in 1.1 and 10.7% of wild Muscidifurax raptor collected on New York dairy farms in 1991 and 1992. Higher (84%) levels of infection were observed on farms where infected M. raptor from a commercial insectary were released. Adult M. raptor became infected (100%) after feeding on spore suspensions of N. muscidifuracis, whereas house fly (Musca domestica L.) adults and larvae did not. Adult M. raptor became infected (16-25%) after feeding on infected parasitoid immatures within host puparia. Transmission rates of N. muscidifuracis increased with increasing parasitoid crowding levels because of cannibalism by healthy parasitoid immatures on infected immatures in superparasitized hosts. Maternal transmission was 100% efficient but there was no paternal or venereal transmission. Disease was reduced 35-93% by immersing infected parasitoid eggs within fly puparia in a 47°C water bath for 30-60 min. Eliminating disease from an established laboratory colony of M. raptor resulted in a 10-fold increase in parasitoid fecundity.