A proteinaceous attractant, prepared by freeze-drying fermented whole egg solids, was found to attractMusca domesticaL. and other synanthropic Diptera. In field experiments, a mixture of 2 g of the attractant and 2 g of dichlorvos sugar-bait increased fly collection two-fold on manure substrate in trays set on the ground and three-fold in suspended bait units over the sugar-toxicant bait alone. This increase was due primarily to the increased response of nulliparous and parous females in which vitellogenesis was about to occur. Attractancy of the bait declined sharply after 48 hours of field exposure.The location of bait stations significantly affected the number, sex ratio, and female age structure of the flies collected. Stations collecting the greatest number of flies were generally situated at the ends of the cage rows in sunlight-shade border areas. Fly collections from areas of greatest fly activity were characterized by a high proportion of hoth nulliparous and parous protein-searching females, and the sex ratio in these high-activity zones approached 1. East-west and north-south preferences of certain female age groups were manifested in the collections.