ABSTRACT Sound plays an important role for toothed whales in foraging and communication. However, little is known about acoustic communication in the toothed whale species that only produce narrow band high frequency (NBHF) clicks, such as the harbour porpoise Phocoena phocoena. To study acoustic behaviour and to quantify the source parameters of porpoise communication signals, the acoustic and swimming behaviour of three adults and one calf were recorded using an array of hydrophones, acoustic tags and an overhead video camera. We tested the hypothesis that different behavioural interactions between porpoises involve specific click patterns for communication and measured the source characteristics of these click patterns to estimate the active space of porpoise click communication. Our results provide strong evidence that porpoises communicate acoustically using specific patterns of clicks with source properties comparable to normal echolocation clicks, and that they employ stereotyped aggressive click p...