Female hamsters [Mesocricetus auratus] emitted high-frequency sounds at average intensities of 53 dB (SPL) [sound pressure level]. These calls tended to be 80-100 ms long and to emphasize frequencies of 34-42 kHz. Female ultrasounds typically included rapid fluctuations in frequency and amplitude. Male hamsters also emitted high-frequency vocalizations, with dominant frequencies of 32-38 kHz and average durations of 70-150 ms. Although male calls generally included fewer rapid changes in amplitude and frequency than did female calls, male call structures depended on contextual factors. Calls produced by males in the presence of estrous females tended to have lower minimum frequencies, higher maximum frequencies, longer durations, and fewer rapid frequency changes than calls by solitary males. Both sex differences and situational factors influence the structures of hamster ultrasounds. The frequency and amplitude changes typical of calls by females and solitary males should facilitate the localization of a calling individual over moderate distances. Calls by males in the presence of females should be more difficult to localize and might operate over shorter distances to serve a different social function.