Large numbers of Manx shearwaters were caught at night at the breeding colony on Skokholm Island, Pembrokeshire, Wales. Most of the birds caught were of known age. In 1973 and 1974 the weight of all age groups was highest in March and then declined to a minimum in June and July. Weight increased slightly in August. It was generally true that the older a bird was, the heavier it was in any particular month, and this effect appeared to hold good until the birds were eight to ten years old. Unlike weight, bill and wing length did not alter with age. It is suggested that, up until an age of about five or six years old, the greater weight of older birds may be due to higher feeding efficiency. This result is therefore related to hypotheses concerning the delayed onset of breeding shown by many seabirds. The seasonal pattern of weight change may reflect the severity of the prevailing feeding conditions.