Abstract: The distinctive Somali Arid and the East African grassland faunas approach each other to the north of the Kenya Highlands and along the Tana River. Both have unique endemics and many counterparts at the species and race level. Mostly these latter remain isolated but there are instances of intergradation and range overlap without interbreeding. An interesting case of the latter, that of the zebras Equus grevyi and E. burchelli, is analyzed here. The unusual feature of the zebra overlap is that mixed herds are formed. These maintain a level of constancy from day to day, and are not disrupted during flight. Mutual tolerance is marked. While mixed herds occur all year, the composition varies from season to season and district to district. Migrations may take place conjointly. Outside of the overlap zone habitat differences are marked. The mixed zebra herds of Kenya are contrasted with the zebra-wildebeest associations of the Kruger Park, South Africa, and it is shown that the former have greater cohesion and constanc