Black fly adults were permitted to engorge on known hosts then retained for varying periods of time in a controlled laboratory incubator or under field conditions. The blood source was identifiable in all instances by precipitin tests conducted on smears 24 hours after the blood had been imbibed. Seventy-four per cent of the meals from flies held in incubators and 60% from flies held outdoors were identifiable after 32 hours, but after 40– and 48–hour intervals few meals were identifiable. The hosts used were horses and guinea pigs. It is tentatively concluded from a comparison of data and from field- and incubator-held flies that temperature influences the rate of digestion.