Underwater impulsive sounds may affect the behavior of harbor porpoises within a certain distance from the sound source. To determine the 50% threshold received sound exposure level (SEL) of an impulsive sound that causes a brief behavioral response (a sudden change in swimming speed and/or direction, similar to a startle response) in harbor porpoises, a male harbor porpoise was exposed once every 3 min to a single impulsive sound: a synthetic exponential pulse with a 5 ms time constant, reproduced by an underwater loudspeaker in a pool (resulting in a signal duration of 10 ms). The sound was transmitted at seven source levels, which were expected to cause brief responses in 10 to 90% of exposures. During each transmission, the harbor porpoise’s behavior was observed, and the presence or absence of a brief response was recorded. A 50% brief response rate was observed at a received SEL of 92 dB re 1 μPa2s, and a zero-to-peak sound pressure level of 122 dB re 1 μPa. The present study suggests that a single impulsive sound as used in this study does not cause a brief response in harbor porpoises at SELs below around 65 dB re 1 μPa2s. The duration and spectrum of impulsive sounds change with distance to the sound source, and such changes may affect the responses of wild harbor porpoises.