The construction of the Horns Rev II wind farm is accompanied by a marine mammal monitoring programme. As a part of that study, underwater noise was measured on the installation of pile J2. During pile driving, a maximum peak level of 195 dB and a maximum SEL of 176 dB were observed at 720 m distance from the pile. At 2300 m distance, respective values were 180 dB for the peak level and 164 dB for the SEL (all sound levels re 1 μPa). The spectral maximum of the pile driving noise was in the frequency range 80 – 200 Hz.