Responsiveness of three species of pinniped to two types of brief (5-s duration) underwater sound was investigated. The experimental stimulus was a broadband pulsed sound (peak levels of 110--135 dB re: 1 (mu)Pa). The control stimulus was a frequency sweep (rms level of 125--130 dB re: 1 (mu)Pa). Reactions were classified as avoidance or approach. Two California sea lions and a harbor seal showed no avoidance reaction to either stimulus. A northern elephant seal showed a powerful avoidance response to the pulsed sound and showed little reaction to the frequency sweep. The sea lions and harbor seal clearly showed habituation to both sounds. However, rather than showing habituation, the elephant seal showed sensitization to the experimental stimulus, as well as to the entire experimental setup. That is, as the experiment progressed, avoidance response to the signal was faster and duration of post-stimulus haulout behavior increased. In fact, following successive presentations of the pulsed sound, it was often not possible to coax the elephant seal back into the water. The results suggest that there are species-specific constraints on habituation to certain types of environmental stimuli.