The average generation interval of 3,53 years in S.A. Mutton Merino stud flocks in l96l/62 is the result of an average ram-age of 3,57 years and an ewe-age of 3,49 years .In 1971/72 the average age of rams and ewes was 3,86 and 4.13 years respectively with an average generation interval of 4,00 years. The extending of the generation interval is mainly a result of an increase of 0,64 years in the average age of ewes. The most important factors leading to the latter are a smaller annual replacement of ewes and the first mating of young ewes at a later age. The generation interval of different strata of the breeding structure could not show a clear relationship between generation interval end the relative importance of the flock.