When parasitized house fly pupae were placed at 8 different constant temperatures in the laboratory, the parasitoidSpalangia endiusWalker, failed to develop completely at 12.8 and 15.6°C. The length of successful development ranged from 66 days at 18° to 17 days at 35°C. Another fly parasitoid,Muscidifurax raptorGirault and Sanders, failed to complete its development at 12.8 and 35°C, but generally developed faster thanS. endius.Under field conditions, parasitoid developmental rates varied on a seasonal basis and ranged between 21 and 183 days forS. endiusand from 19–126 days forM. raptor. Developmental rates of both species were similar during summer, butM. raptordeveloped at a significantly faster rate during cooler months.An algorithm was used to predict development of parasitoids by providing simulation of responses to constant and variable temperatures. High correlation coefficients between predicted and observed developmental rates were obtained with both parasitoid species.