The tables published by Ewbank, Phillipson, Whitehouse & Higgs (1964) permit the determination of the age of sheep mandibles up to 24 months, after which eruption is complete and wear is in progress on all the teeth. The attempt now made to extend age determination to older animals depends on the condition of homogeneity in initial size and rate of wear among sheep teeth from sites with a wide range of archaeological dating. So far as the Thames Valley is concerned, this condition appears to be satisfied at least from the Iron Age to the Medieval Period, and the author has yet to encounter a site where departures from the norm are sufficient to invalidate the ages tentatively established. There is of course no means of making an independent check of the ages so determined and the only test is the degree of internal consistency obtained between a set of jaws believed to be of the same age. When seasonal breeding and seasonal killing of sheep coincide, the result is the simultaneous death of a group of animals of substantially the same age and the test can then be applied. It is believed that a jaw containing the three molar teeth can be aged to a month with 80% confidence and isolated teeth to within 3 months, with the exception of worn Ml and P4. P3 and P2 are virtually useless for dating after eruption, as are m2 and ml. Incisors and canines are of too infrequent occurrence to be of service. Ages over 60 months become progressively more unreliable. The initial calculation of the rate of wear was made on a group of 30 mandibles and maxillae from the Iron Age hill fort at Blewburton (unpublished excavation), the ages of which, all being under 36 months, were calculated from the data of Ewbank et al. It is impossible to make a direct measurement of the unworn height of a bovid tooth because the pulp cavity remains open and the roots are unformed after wear on the crown has already begun. It was therefore necessary to take a selection of slightly worn teeth with closed roots and estimate a notional initial height from the top of the unworn crown to the point at which the roots divide by direct comparison with a group of slightly younger teeth which were unworn. Knowing the age at which the teeth in the complete sets had come into wear and the age at which wear had ceased due to the death of the animal, it was possible to estimate the rate of wear in mm per month. This estimate was then used to compute ages for the remaining older jaws and loose teeth from the same site. When these were plotted in histogram form they were found to produce a series of peaks at intervals of 8 months. The rate of wear was therefore recalculated using the data of Silver (1969) for ages at which teeth erupt in unimproved and half-wild sheep. These, being about 50 % greater than those of Ewbank et al. (1964), generated a much more plausible 12-month cycle. The method was then tried on material from other sites in the Thames valley (all as yet unpublished) with similar results, bringing the total sample size up to 83 mandibles and maxillae.