Fly pupae, primarily house fly,Musca domesticaL., and stable fly,Stomoxys calcitrans(L.) (Diptera: Muscidae), were collected from a poultry house in northern Indiana, and parasitoid wasps developing in them were allowed to emerge. Fly pupae density, relative species abundance, and size varied with collection date. Four species of parasitoid wasps were reared from the fly pupae:Spalangia endiusWalker,S. cameroniPerkins,S. nigroaeneaCurtis, andMuscidifurax raptorGirault&Sanders (Hymenoptera: Pteromalidae). There was some evidence of differential host species and size usage among the wasp species; but the differences were small, and there was considerable overlap. Percentage of parasitism was independent of host species forS. endiusandS. cameroni, butM. raptoremerged from house fly pupae more frequently than expected by chance and from stable fly pupae less frequently than expected.S. endiusemerged from larger hosts than didS. cameroni, despiteS. endiusbeing on average smaller thanS. cameroni. This is the first study to examine differences of host size among these parasitoid wasps.