Signals used in naval and long-range fish detection sonar often contain harmonics which may influence the behavior of harbor porpoises (Phocoena phocoena) more than the lower fundamental frequencies, so the behavioral effects of 25-kHz FM signals with and without high-frequency side bands (71 and 121 kHz) on a harbor porpoise were quantified in a pool. Sequences of sonar signals were transmitted at four average received sound pressure levels (SPLsav.re.) to determine the dose-response relationship. Sequences, lasting 30 min, consisted of 50 ms, 25.5- to 24.5-kHz down-sweeps (2-s pulse interval), with and without side bands, with identical SPLs at the fundamental frequency. Behavioral effects were quantified as the harbor porpoise’s distance from the transducer, respiration rate, number of jumps, and relative swimming speed. The distance to the transducer changed little in response to the sounds. Respiration rate increased with increasing SPL to maximum respiration rate increases of ~39%, accompanied by jumps, for sweeps with side bands at an SPLav.re. of 148 dB re 1 μPa. At similar broadband SPLs, signals with side bands had a greater effect on the harbor porpoise’s behavior than signals without. Side bands may influence behavioral responses, both by making sounds more audible and by affecting the way sounds are perceived by harbor porpoises. Mitigation of active sonar impact could be achieved by reducing the level of side bands (or harmonics). This would benefit species, such as the harbor porpoise, that are likely to have lower hearing thresholds for the frequencies of the harmonics than for the fundamental frequency.