Wistar rat pups are generally more “vocal” than Lister pups; the ultrasonic calls that they produce are usually louder and emitted at a greater rate than those of Lister pups of the same age and studied under similar conditions of isolation or handling. These differences in ultrasonic behavior are apparently not related to differences in the development of homiothermy which follows a similar course in both strains. The calls of infants of the two strains do not show any marked differences in duration or frequency pattern that are consistent over age or treatment condition, but the Wistar pups appear to show greater changes with age than the Lister pups in certain parameters of the calls, while the latter strain show greater changes with treatment condition. Similar strain differences in the rate and intensity of calling were seen during aggressive encounters between adult males, but these cannot be related to differences in aggressive behavior. The strains differ in the duration of both the long “22 kHz” calls (Lister greater than Wistar) and the shorter “50 kHz” calls (Wistar greater than Lister), as well as in the minimum frequency of these shorter calls (lower in Wistar than in Lister rats).