In a 3-year study it was found that during the summer total fly populations in dairy barns are variable but as the season progresses they generally increase. This pattern is not true for all fly species. The house fly, Musca domestica L., the most important fly found in dairy barns, does increase greatly late in the season, but, Fannia spp., on the other hand, are abundant early in the season but disappear as weather becomes warmer. With increasing temperatures Fannia spp. tend to rest more and when this occurs residual insecticides are effective against them. Stomoxys calcitrans (L.) are important locally where there are suitable breeding conditions. In any one barn the fly population may be highly variable depending on such factors as breeding conditions and use of insecticide.