Manure disposal can be a serious environmental problem in poultry-cage facilities for egg production. Flies can degrade manure and they offer a possibility of recycling products. All flies studied were inadequate to degrade accumulattons of anaerobic manure. Among several species of muscoid flies testedMusca domesticaL. was most effective in degrading aerobic manure. Different strains thrived in manure variously according to their general vigor. Poultry manure provides an adequate source of protein for egg maturation forM. domesticaand it is a suitable oviposition medium except when a more attractive medium is present. Manure is an adequate medium for maggot culture, although of lower quality than a standard medium; its quality diminishes with age.As manure is biodegraded, metabolic activity of contained organisms increases to a plateau, ammonia is produced, nitrogen is lost, and pHincreases. Bacteria, rather than fungi and yeasts, are primarily responsible for these changes, but metabolism of bactcria and developing maggots interact. Some bacteria may retard fly development. Fly larvae perform a conspicuous role in aerating medium to favor aerobic bacteria and to hasten loss of nitrogen and moisture. The design of batch or continuous systems of manure degradation by flies should include developmental and behavioral characteristics of insects appropriate to the system.