1.Surveys of human excrement in privies at Phoenix, Arizona, during 1951–52 demonstrated that Musca domestica breeds readily in this medium. Of 193 privies positive for fly breeding, 44.6 per cent were infested with house fly larvae. Larvae of Sarcophaga, Muscina, and Hermetia occurred in 31.1, 16.1 and 10.9 per cent of the positive samples, respectively.
2.Samples of human excrement from chemically treated privies in an urban area showed infestation rates of 31.9 per cent for M. domestica and 20.7 per cent for Muscina spp. whereas those from privies in an untreated zone displayed rates of 3.6 per cent and 59.0 per cent for M. domestica and Muscina respectively. No conclusive data explain this reversal in species predominance but several possible causes are discussed.