We recorded underwater vocalizations of captive and wild Florida manatees (Trichechus manatus latirostris) to assess variability in acoustic structure of their sounds and to test hypotheses regarding the importance of specific acoustic traits in individual distinctiveness and in certain behavioral contexts. Manatees use vocalizations to maintain contact when in groups. The highest rates of vocalizing occur during antiphonal calling between females and calves. Vocalizations are complex, single-note calls with multiple harmonics, frequency modulations, nonharmonically related overtones, and other nonlinear elements. We measured 6 acoustic variables and found that individuals varied significantly in fundamental frequency, emphasized band, frequency range, and call contour (the overall pattern of complexity in frequency modulation). These traits did not vary within individuals on different dates or when manatees were alarmed and fleeing. Individual fundamental frequencies ranged from 1.75 to 3.90 kHz, and were negatively correlated with body size. Little sound energy occurred above 18 kHz in 502 call notes of 6 captive manatees sampled with a recording oscilloscope. Presence of harmonics and call duration differed by date and manatees emitted longer calls when fleeing disturbance. Call duration varied from 118 to 643 ms (geometric mean = 271 ms, 95% confidence limits = 264, 279 ms) in a sample of 479 vocalizations we recorded from 14 individuals. The maximum call duration recorded over the entire study was 900 ms. Females and calves responded only to each others' vocalizations when rejoining a group after brief separations, strongly suggesting individual recognition by sound. Structural complexity in the calls of manatees is similar to that in other sirenians, and may reflect their auditory capabilities and the unique physical properties of sound in shallow water.