Abstract Timing of birth of domestic animals is regulated by environmental and genetic variables, and can also be manipulated by the herder. For this reason it is difficult to predict birth distribution in a prehistoric context. Birth seasonality of prehistoric sheep from the site of Kasteelberg, South Africa, was investigated by high resolution intra-tooth analysis of enamel oxygen isotope ratios. The results show two birth seasons, probably in autumn and spring. This may reflect females lambing more than once a year, or subdivision of the flock into groups where births were scheduled at different times of the year. This result has important implications for food availability throughout the year, the role of sheep in the subsistence economy, residential mobility strategy of the human community and use of sheep mortality profiles for interpretation of season of occupation of archaeological sites.