The loud sounds produced under water during offshore percussion pile driving for the construction of wind turbines may affect harbor porpoises (Phocoena phocoena). Kastelein et al. (2013b) exposed a porpoise in a quiet pool to playbacks of underwater pile driving sound at several mean received sound pressure levels (SPLs; range: 130 to 154 dB re 1 mu Pa) and suggested that harbor porpoises at sea swim away from offshore pile driving locations (moving tens of km), thus reducing their received SPL. The speed at which they swim both determines the acoustic exposure and impacts the energetic costs of a behavioral response. Therefore, information on swimming speed is important for estimating the potential impact of pile driving sounds on the hearing, the energetics, and the population dynamics of harbor porpoises. The video recordings from the Kastelein et al. (2013b) study were analyzed for swimming speed. During quiet baseline periods, the mean swimming speed of the porpoise was 4.3 km/h, and he swam a mean distance of 2.2 km in 30 min. Even at the lowest SPL tested (130 dB re 1 mu Pa), his mean swimming speed was significantly greater than during baseline periods. At the highest SPL (154 dB re 1 mu Pa), his mean swimming speed was 7.1 km/h, and he swam a mean distance of 3.6 km in 30 min. Swimming speed did not decline significantly during the 30-min test periods, and a speed of similar to 7 km/h appears to he sustainable for harbor porpoises.