Average values of morphological parameters for mature harbour porpoise (Phocoena phocoena) are presented. Variation in these parameters with body length was examined for a sample of 35 porpoise. Several parameters including fineness ratio, fluke aspect ratio, and the ratios of fluke area:body surface area and (body surface area)1/2:(body volume)1/3 were found to vary with body length. Fluke aspect ratio was inversely proportional to fluke sweep angle. The finding that many morphological parameters vary with length for harbour porpoise indicates the dependence of these parameters on age; this may also be true for other cetacean species.