The effects of an undetermined species of Nosema on fitness of the muscoid fly parasitoid Tachinaephagus zealandicus were examined in the laboratory. Infected female parasitoids that were given honey and water lived about one-half as long as uninfected parasitoids under these feeding conditions. Effects of infection on longevity were strongest at 30°C; infected and uninfected females lived 2.8 and 8.7 d, respectively. Infected and uninfected parasitoids that were given only water had similar longevities, but water-only–fed parasitoids had much shorter lifespans than honey-fed parasitoids at all temperatures. Infection did not result in significant lengthening of development times of immature stages, with male and female parasitoids completing development from egg to adult in ≈23, 33, and 60 d at 25, 20, and 15°C, respectively. Overall emergence of uninfected parasitoid adults was 16 times greater than infected parasitoids at 15°C. Emergence of uninfected parasitoids was 11 and 3 times greater than infected parasitoids at 20 and 25°C, respectively, and sex ratios of emerged adults were significantly more male-biased in infected parasitoids at these temperatures than among uninfected parasitoids. Dissections of uneclosed puparia revealed that many infected parasitoids completed development to the adult stage but did not successfully emerge from host puparia. Infected and uninfected females killed similar numbers of hosts (70–75 house fly or Sarcophaga bullata larvae killed per group of five females in 24 h). Uninfected females parasitized significantly more house fly larvae (59.7) and produced more than twice as many adult progeny (311.1) as infected females (34.1 hosts parasitized, 138.3 progeny produced). Infected females parasitized about as many S. bullata hosts as uninfected females and produced slightly fewer adult progeny (588.2 and 460.1 progeny per group of five uninfected and infected females, respectively). In tests with individual females given house fly hosts daily throughout life, uninfected and infected parasitoids had similar longevities (3.9 and 3.7 d, respectively), but uninfected parasitoids produced 2–5 times as many adult progeny.