Ultrasonic (>20 kHz) vocalizations were recorded from two species of prosimian primates, Microcebus murinus and Cheirogaleus medius. Three types of ultrasonic calls were emitted by isolated or paired M. murinus: 1) “chirps,” short-duration (30 msec) calls with a sharp onset at 40 kHz followed by rapid and steep declines in frequency and intensity; 2) “twitters,” characterized by a brief pulse at 25 kHz prior to the onset of repeating, frequency-modulated units; and 3) “long whistles,” monotone signals with a frequency of 20 kHz. Partially ultrasonic “whines” were recorded from C. medius during interactions between mothers and infants. These calls began with a short, high-intensity component at 25 kHz and then dropped gradually in frequency before leveling off at 15 kHz. Study of the acoustic behavior and the structure of vocalizations in these species may lead to a better understanding of animal communication at high frequencies.