Aim The study tests the hypothesis that land-use changes in Narok District have had an impact on the wildebeest population [Connochaetes taurinus mearnsi (Burchell)] in the northern part of the Serengeti–Mara ecosystem.Location The Serengeti–Mara ecosystem is a vast area of rangelands, straddling the Tanzanian–Kenyan border in East Africa. The area is home to some 1.3 million wildebeest, of which some 30,000 animals currently reside in the Kenyan part of the ecosystem.Methods We analysed the temporal changes in the wildebeest population in the Kenyan part of the Serengeti–Mara ecosystem and their relationship with possible driving forces of change, such as rainfall, normalized difference vegetation index (NDVI), livestock numbers and land-cover changes. Changes in the spatial distribution of wildebeest for three periods were compared with spatial changes in livestock distribution and land cover. The analyses were repeated for the Tanzanian part of the ecosystem and results compared. We thus tested the relative importance of land-use changes among the different potential driving forces of change in the wildebeest populations.Results The wildebeest population in the Kenyan part of the Serengeti–Mara ecosystem declined drastically over the past 20 years and is currently fluctuating around an estimated population of 31,300 animals, which is about 25% of the population size at the end of the 1970s. The wildebeest population in the Kenyan part of the Serengeti–Mara ecosystem has, over the last decades, been controlled by food supply during the dry and the wet seasons. Expansion of large-scale mechanized wheat farming in the Loita Plains since the early 1980s has drastically reduced the wildebeest wet season range, forcing the wildebeest population to use the dryer rangelands in the south-eastern part of the Loita eco-unit, or to move to the Mara eco-unit, where competition with cattle is higher. The expansion of the farming area has not influenced the size of the total cattle population in the Kenyan part of the study area, nor its spatial distribution. The much larger migratory wildebeest population of the Serengeti, in Tanzania, has not been affected by a downward trend from the late 1970s and is regulated by food supply in the dry season (Mduma et al., 1999). Around the Serengeti, in Tanzania, land-use changes are much less widespread, occur at a lower rate and affect a much smaller area compared with the Kenyan part of the ecosystem. Moreover, land-use changes around the Serengeti have taken place away from the main migration routes of wildebeest.Conclusions Over the last decades, the decline in the Kenyan wildebeest population did not seem to affect the much larger Serengeti wildebeest population. However, if more land were to be converted to large scale farming closer to the Masai Mara National Reserve, the dry season range for both the Kenyan and the Serengeti population would be reduced. This might have serious consequences for both populations and therefore for the entire Serengeti–Mara ecosystem.