The characteristics of echolocation signals used by a harbour porpoise (Phocoena phocoena) during a target detection experiment are described. A water-filled steel sphere (either 5.08 or 7.62 cm in diameter) was placed at distances of 12, 14, 16, 18 and 20m from a harbour porpoise stationed in an underwater hoop. Detection was determined by a go/no-go procedure. Target trials were alternated with no-target trials according to a pseudo-random schedule. The harbour porpoise detected the large sphere at all distances, while the small sphere was detected up to 14 m. It used clicks with less energy when detecting the large sphere at all distances. The porpoise used, on average, 13 clicks to make a decision for the large sphere, which was significantly fewer compared to the small sphere (34 clicks) and to no-target trials (37 clicks). The mean interval between clicks was almost constant (about 59 ms) and independent of distance to target. The individual pulse trains showed two kinds of small-scale variations in click intervals: (1) jittering, which could be a way of avoiding range-ambiguous interference and (2) cyclic modulation. The mean source levels for all targets and all distances ranged from 157 to 169 dB re 1 µPa (p-p). The clicks were on average 77 µs in duration and had a peak frequency of 131 kHz. A low amplitude pre-click was seen prior to the majority of the clicks recorded. The pre-click occurred on average about 270 µs before the main click, regardless of target present or not, and was correlated temporally and spectrally to the subsequent main click. A pre-click has not previously been reported or found in the signals of three other harbour porpoises and may be an anomaly in this individual.