Helicopter Long Range Active Sonar (HELRAS) is deployed from navy helicopters to detect submarines. Its worldwide use by NATO navies is expected to increase due to the increasing need to detect sub-marines at greater distances. The 1.33 to 1.43 kHz signals vary in spectrum and duration depending on the circumstances. HELRAS sonar may affect the behavior of harbor porpoises (Phocoena phocoena) within a certain range. To investigate this, a harbor porpoise in a large pool was exposed to five HELRAS signals with different spectra, each of 1.25 s duration, at multiple sound pressure levels (SPLs), which had been determined in a pretest. During each transmission, the presence or absence of a brief behavioral response (defined as a sudden change in swimming speed or swimming direction during sound emission) by the harbor porpoise was recorded. A 50% response rate was observed at mean received SPLs (averaged over all measurement points in the pool) of between 124 and 140 dB re 1 µPa, depending on the signal. A single emission caused no response at received SPLs below around 102 dB re 1 µPa (sound exposure level: 103 dB re 1 µPa2s). The highest level needed to induce a brief response in the harbor porpoise occurred when it was exposed to a down-sweep without harmonics. Therefore, of the five tested signals, this signal will presumably have the least effect on harbor porpoises, and its use could help to minimize the potential effects of HELRAS sounds during peace time exercises.