A total of 528 samples totaling 37,639 house fly and 11,447 stable fly pupae were collected from 34 confined bovine feedlots and dairies in eastern Nebraska and examined for pteromalid parasites. Parasitism averaged 14.2% for house flies and 7.9% for stable flies. Levels of parasitism were consistent from feedlot to feedlot but were higher on the dairies. Seven parasite species were identified:Muscidifuraxspp. made up 56%,Spalangia cameroni11%,S. nigroaenea31%,S. nigra1%, and others 1% of the 5,881 parasites attacking house flies;Muscidifuraxspp. made up 25%,S. cameroni26%,S. nigroaenea44%,S. nigra4%, and others 1% of the 979 parasites attacking stable flies.Muscidifuraxspp. were recovered significantly (P= 0.01) more often from house flies, andS. nigrawere recovered significantly (P= 0.01) more often from stable flies. Parasite activity was initially observed during the first week in June and increased to a mean of 20.9% during October. The activity and the species of parasites attacking fly pupae were influenced by the habitat in which the hosts were found