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Absolute thresholds were determined for pure tones of frequencies 100 Hz-40 kHz by the method of constant stimuli using a two-bar operant technique. Lemurs of three subspecies were trained to press a white bar to turn on a tone lasting 5 sec. During the tone, a single press on a second (black) bar resulted in food reward. Failure to respond simply terminated the trial. Response in the absence of tone was scored as a false positive response and resulted in a «-sec shock of from 0.6 to 1.3 mA. Thresholds, based on a response probability of 0.5, indicate a lesser sensitivity in the low and middle frequencies than found in the anthropoid primates, but an apparent extension of sensitivity into the higher frequencies beyond the upper-frequency cutoff points found for the anthropoids. I. PROCEDURE A. SubjectsSix lemurs, males and females of each of three species (lemur catta, lemur macaco fulvus, and lemur macaco macaco), served as subjects. Their weights, breeding behavior, and dentition indicate that three were adult and three adolescent (see Table I). All were healthy; one (No. 2023) was extremely fat and had also been previously treated with antibiotics for a jaw infection. The animals were deprived of food and received ad lib water during training and testing. The extent of deprivation may be estimated from the weight data included in Table I. B. ApparatusThe animals were trained and tested in a small cage inside a double-walled IAC chamber. Standard audiometric and programming equipment was used to generate and control the sound stimulus, food, and, later, shock. A Western Electric 555 speaker produced