Population parameters which are derived from a study of marked birds on Skomer Island, Wales, and from ring recoveries are presented. Breeding success on Skomer averaged 0.7 chicks/pair, and in 2 yr adult survival averaged 91.5%. Guillemots returned to Skomer in their 2nd yr; prior to breeding, birds congregated in clubs. Guillemots 1st bred at 4 yr old, but most of them probably breed for the 1st time in their 5th yr. Two different estimates for the survival rate of birds up to 5 yr old were 16 and 20%. Juvenile survival calculated from ring recoveries varied from 27% for southern England, where numbers are stable or decreasing, to 41% in northeast Canada, where numbers have increased. Between 1972-1977 the Skomer guillemot population remained at a similar level. If adult survival is 91.5% and 100 pairs of guillemots produce 70 chicks, 17 (24%) of these must survive to breeding age to maintain a stable population. Factors influencing population size are discussed.