In humans, sound perceived as speech is processed preferentially by the right ear and the left hemisphere of the brain. Among animals, such an advantage of one hemisphere (lateralization) in processing communication sound from other members of the species has so far been demonstrated only in macaque monkeys. I report here that in the house mouse, which has a very much less elaborate forebrain than man or macaque monkey, the ultrasonic calls that are emitted by young mice to evoke maternal caring behavior are preferentially recognized by the left hemisphere. In females with no experience of pups, which have been trained to respond to the same ultrasonic calls by conditioning, no advantage for one hemisphere is detected. The results suggest that lateralization of this function evolved early in mammals and emphasize that an innate predisposition for perceiving communication sounds is connected with a left-hemisphere advantage in processing them. This experimental system is a readily-available animal model for studying lateralized auditory brain functions.