In outdoor cage studies undertaken in the late summer of 1973, a method of estimating the absolute density of a stable fly (Stomoxys calcitrans (L.)) population was developed by correlating the average number of insects feeding or resting on a calf to the total number in the cage. Under the same cage conditions, where emigration, immigration, and to some extent, predation have been excluded, the daily mortality rate of normal stable flies marked with a fluorescent dye ranged from 25.3% to 27.3% dependent upon the sampling technique. Marker dyes have a deleterious effect on survival but surprisingly gamma irradiated flies succumbed at a lower rate than normal individuals.