Ultrasonic vocalizations of infant rodents are used in developmental studies and for investigating the effects of drugs or environmental pollutants. Few studies, however, have analyzed the frequency characteristics of these ultrasonic vocalizations. This study investigates the physical and vocal development of infants (1–14 days old) of the short-tailed field vole, Microtus agrestis, under 2 conditions of isolation: at 23°C immediately after being isolated from the nest or at 23°C after 10 min of isolation at 27°C. Seventy-three percent of the infants vocalized, and there was great variation among calling infants in the number of vocalizations emitted. More infants, especially males, called during the 2nd period of isolation than during the 1st, and latency to call increased with age in males. The ultrasonic vocalizations were comparable to those recorded from North American voles. Vocalizations were classified into 7 categories on the basis of characteristics of the fundamental frequency. Simple calls were emitted most commonly and became more frequent in older infants, whereas the proportion of calls with a down-sweep in frequency and audible clicks decreased with age. The emission of ultrasonic calls, therefore, appears to reflect changes both in physical development of the infants and in their external environment.