Naval sonar systems produce signals that may affect the behavior of harbor porpoises (Phocoena phocoena), and behavioral responses may be influenced by the received signal and by the signal-to-noise ratio (SNR). The widely used AN/SQS-53C hull-mounted sonar system produces 1,600-ms signals with three components in the ~3.5 to 4.1 kHz band. To investigate the effect of the SNR on respiration rate (an indicator of the behavioral response to the sonar signals), two porpoises were exposed to 30-min playbacks of 53C sonar signals (96% duty cycle) in noise corresponding to sea state 6 conditions. Two signal-to-noise conditions were tested: 53C sonar was produced at an SPL of 117 dB re 1 Pa (SNR = 49 dB re 1 Hz), which caused no response; and at 122 dB re 1 Pa (SNR = 54 dB re 1 Hz), which caused an increased respiration rate in both porpoises. In quiet conditions, one of the porpoises had responded to the signal at approximately the same SPL (Kastelein et al., 2018). These measurements suggest that the behavioral responses of harbor porpoises to naval sonar signals are unaffected when the ambient noise is similar to that of wind noise up to sea state 6.