Survival of Musca vetustissima Walker from egg to adult in nutritious cattle dung from pasture irrigated during the dry south-western Australian summer was measured in the field. When drowning was prevented by placing dung on a low embankment, survival was high for only the first two months of the fly season, which included the first month of the five-month irrigation period. Substantial emergence of wild M. vetustissima adults from naturally- infested dung occurred only around that time. Survival was so low during the last four months of irrigation, even in the absence of mortality caused by drowning, that irrigated pasture areas in south-western Australia are unlikely to have significantly greater potential for breeding of M. vetustissima than non-irrigated pastures during most of the summer.