Harbour porpoises Phocoena phocoena are unintentionally caught in gillnets. Although the effectiveness of by-catch mitigation measures has been evaluated many times, only a few studies have investigated the behavioural patterns of the porpoises in relation to the presence of fishing nets. In this study, the reactions of wild harbour porpoises to a modified gillnet were visually observed. The porpoises responded to the net by avoiding it, with reactions being detectable at a distance of >80 m. The number of animals in the pods approaching the net did not affect the behaviour of individuals in relation to the net. These results strongly indicate that porpoises do not usually actively approach gillnets. The by-catch problem seems, therefore, to be caused by individual animals accidentally being caught, likely due to attention shifts or to auditory masking reducing their ability to detect the nets using echolocation.