Four dairies in southern California were sampled weekly from April 1987 through June 1988 to determine the distribution and relative abundance of stable fly,Stomoxys calcitrans(L.), and house fly,Musca domesticaL., pupal parasites. Seven parasite species represented ≍90% of the parasites recovered from stable fly and house fly pupae, and includedSpalangia endiusWalker,Spalangia cameroniPerkins,Spalangia nigroaeneaCurtis,Muscidifurax raptorGirault and Sanders,Muscidifurax zaraptorKogan and Legner,Urolepis rufipes(Ashmead), andAleocharaspp.U. rufipeshad not been previously recorded to parasitize these hosts in California. Most of theU. rufipesrecoveries were between April and June. The relative abundance of parasites recovered from stable flies and house flies on four southern California dairies varied slightly for some species, but the overall rate of parasitism only ranged from 13.4 to 19.9%. A limited number of stable fly and house fly pupal collections from dairies in central California revealed a much lower proportion ofS. cameroniand a higher proportion ofS. endius, when compared with southern California dairies. The majority of stable fly and house fly pupae sampled at four southern California dairies were associated with pure manure substrates. There was no significant difference between the relative abundance of parasites recovered from stable fly pupae sampled from manure, manure plus hay, or wet-hay substrates. Significantly fewerS. nigroaeneaand significantly moreM. zaraptorwere recovered from house fly pupae developing in brewer's grain when compared with manure, manure plus hay, and wet-hay substrates. In pupal samples which contained both stable flies and house flies,S. cameroniwas recovered significantly more often from stable fly pupae. In general, there were no significant differences between stable fly and house fly in parasitism by the seven parasite species.