RAPID population growth is commonly depicted as one of the greatest problems facing modern Africa. For decades, the tendency of birth rates to exceed mortality rates has prompted predictions of land shortage, resource depletion and mass starvation. Underlying causes of high fertility are hypothesized to have been an unusually high demand for human agricultural labour, ‘traditional religious pronatalism’ and a ‘horror of barrenness’, while in some areas the later colonial period saw a shortening of the durations of post-partum sexual abstinence and lactation. Mortality decline from the 1920s is commonly linked to the establishment of cash crop economies, networks of roads and railways, and the diffusion of western medicine, maternity facilities, missionary activity and primary education. Yet the empirical evidence supporting this model of population growth is contradictory. Areas such as Buhaya, Buganda and Bunyoro should have experienced rapid demographic expansion by natural increase in the colonial period according to dominant theories but instead experts in the early decades of this century feared the extinction of the Haya, Ganda and Nyoro. This paper will attempt to explain why population decline among the Nyoro was more severe than anywhere else in colonial Uganda, and probably East Africa.