A method of determining the age of grey seals, using annual rings in the cement deposited on the canine teeth, is described. Using this, several parameters of growth are correlated with age and the normal ages for sexual maturity are established as five years for cows (first pup at six years) and six years for bulls. Bulls normally appear not t o become holders of territory however until they are ten years of age. The two sexes are of very unequal longevity. Few bulls live beyond twenty years, probably none beyond twenty-five; cows do not exceed thirty-five. There appears to be a high mortality (c. 40 per cent) among territorial bulls. The age composition of a breeding colony and the bull to cow ratio are accounted for by the differences in the number of year groups of the two sexes and by the much smaller numbers of bulls in the later groups due to the high mortality. A provisional life table is proposed.