A study of sources and seasonal abundance of the more common muscoid flies in rural areas from May, 1952, through April, 1953, showed that Musca domestica was produced by nineteen of twenty breeding materials throughout the period of the study. No other species approached the housefly in this respect. Two chief categories of animal refuse (excrements and animal pen litter) occurred most frequently in media examined in the towns and were found to contribute 87.1 per cent of all houseflies produced from all types of breeding material. Miscellaneous, animal, and fruit-vegetable wastes were found to occur less frequently and to produce fewer flies. The abundance of Phaenicia spp. (garbage flies) and Sarcophaga spp., while less than houseflies, was markedly uniform throughout the year and is suggestive of the relatively minor importance of environmental factors. These factors are however adverse to houseflies.