Data from 18 272 births were used to determine the seasonality of calving amongst unimproved cattle in three ecologically distinct regions of Swaziland. Cattle were of the Nguni breed; bulls had access to cows throughout the year. In both the Highveld and Middleveld, there was a pronounced unimodal distribution in calving pattern with peak calving occurring during the period October – December (57.0 and 54.9% of births, respectively). In the Lowveld, the main calving period was November – January (51.0%) but there was also a smaller peak lo July (8. 9%). Monthly calving frequency was correlated with previous monthly rainfall records and most variation was accounted for by rainfall 10 months earlier in both the Highveld and Middleveld (79 and 50% of variation, respectively). No significant relationships between calving frequency and rainfall could be demonstrated in the Lowveld data. These findings are discussed in the context of the nutritional characteristics of the forage available in the three regions.