The harbour porpoise, one of the smallest marine cetaceans of the Northern hemisphere, is a frequent victim of the continental shelf bottom-set gill-net fisheries with many thousands of animals being killed annually. The reasons for this high mortality are not clear as these animals possess a sophisticated active sonar system with which they detect, track and intercept small fish targets. The performance and limitations of this animal's sonar are therefore being studied in order to develop techniques that will minimise cetacean/ gillnet interactions. Sonar source levels and spectra were carefully determined in an enclosed environment from signals transmitted by two juvenile animals. This paper examines these signals and, in the context of the detailed structure of the vestibular air sacs, presents a new hypothesis suggesting that a passive biological mechanism exists in these animals which can explain the formation of their unusually narrow band echolocation signals. The knowledge gained to date in this and associated studies suggests that the harbour porpoise operates a relatively low power, short range sonar capable of detecting (ingestible size) individual fish out to a maximum range of about 30 m. The strongest acoustic emissions of this animal are expected to be around 120-140 kHz with Source Levels, in open water conditions, around 160-170 do re 1µpa at 1m. These sonar signals will be confined by the projecting aperture to a very directional beam pattern (approaching 8° in azimuth and 14° in elevation) projected directly ahead of the animal. The spectrum of the harbour porpoise's sonar pulse exhibits a single power peak and the -3 dB bandwidth is typically around 13 kHz. The harbour porpoise possesses a much shorter range, lower power, sonar than its larger delphinid cousins. In addition its more limited bandwidth may be expected to generate target echoes with less spectral coloration which in turn may impair its ability to correctly classify unfamiliar hazardous targets such as fishing nets from a safe distance.