The blubber thicknesses of 5 male and one female harbour porpoises were measured ultrasonically in 9 positions on the body over various periods of time. Most animals were immature and all grew during the study. The blubber was not evenly distributed over the body, and there were individual differences in blubber thickness. Most males had similar blubber thickness distributions; moving caudally, the thickness increased towards the beginning of the dorsal fin (around 25 mm) and then decreased towards the tailstock (around 17 mm). In the female, the blubber layer thickness varied less between positions than in the males, but was also thinnest laterally on the tailstock. At one time she experienced a large drop in mean blubber thickness and weight in a short time period (maximum rate of decrease: 6 mm blubber/week), despite a constant food intake and good health. Her energy requirements probably increased due to an increase in activity level. After the sudden weight drop had been noticed, she was fed ad lib. Her food intake increased by 70%, and her body weight and blubber thickness quickly returned to normal levels (maximum rate of increase: 7 mm blubber/week).