This article attempts to analyze and explain the differences in fertility between the various regions of Kenya. Until recently, the country has experienced some of the highest fertility levels in the world, but lately the overall fertility has been declining fairly rapidly, especially in the central parts of the country. Kisii District in western Kenya has been selected for an in-depth analysis of the persistence of high fertility and its relationship with the socio-economic characteristics of the area. Demographic transition theory assumes that fertility transition is determined by the economic rationality of having children. This rationality is expressed in the direction of the intergenerational wealth flow, which. in traditional societies, is normally from the younger generation to the older. whereas the direction is reversed with economic development and Westernization. Of particular importance are the introduction of a monetary economy and the spread of education. The study finds fertility transition in process at different stages in the different regions of Kenya, depending on the particular socio-economic situation, but these differences are likely to even out in the future