Reproduction data of 839 Dormer and 741 SA Mutton Merino ewes were analysed in order to evaluate certain conventional selection criteria for reproduction rate in sheep. Results indicated that, although a positive response in reproduction rate can be expected for the SA Mutton Merino by using birth type of breeding ewes as a selection criterion, only a marginal response can be expected for Dormer ewes. These differences between breeds are apparently due to differences in the heritability of fecundity (0,26 vs 0,04 for the SA Mutton Merino and Dormer respectively). Owing to the relatively high repeatability estimation for fertility obtained for both breeds in the present study (0,17 vs 0,15 for the SA Mutton Merino and Dormer respectively), the culling of ewes that fail to lamb during their first lambing season is recommended in order to improve, within generations, the reproduction rate. Owing to differences in the repeatability of fecundity between the two breeds, the use of the number of lambs born during the first lambing season as a selection criterion' had a further significant effect on the subsequent reproductive performance of the SA Mutton Merino ewes only. Results further revealed that early reproductive performance after the first two lambing seasons as well as a combination of early reproductive performance and birth type of the ewe can be used effectively as a selection criterion to improve overall reproductive performance in sheep flocks.