The social organization of three populations of 2 species of zebras in Kenya and its relationship to their communication behaviour were studied. Grant's zebras (Equus burchelli bohmi) (GTZ) inhabiting a dry savanna area had a harem-type social organization. Grevy's zebras (Equus grevyi) inhabiting a dry savanna area with many bushes, formed a territorial society (GYZ-1). Grevy's zebras (GYZ-2) inhabiting the same area as GTZ, created groups of a few to 20 males and females, bachelor groups and solitary males in the rainy season, and formed large groups of over 100 males and females in the dry season. The GTZ displayed three types of scent-marking, but GYZ-1 showed seven types, and GYZ-2 four types. In the GTZ the frequency of scent-marking with the urine of males on the urine of females (FUU) in the rainy season, and in the GYZ-1 the frequency of scent-marking with the faeces of males on his own faeces (MFF) during both seasons, were each significantly higher than other types of scent-marking in the respective groups. In the GYZ-2 male/female mixed groups, the frequency of FUU was significantly higher during the rainy season than during the dry season. The frequency of mutual grooming and mutual chin-resting in the GTZ was each significantly higher than that in the GYZ-1 or GYZ-2. Mutual chin-resting was occasionally observed in the GYZ-2. Neither mutual grooming nor mutual chin-resting was observed in the GYZ-1. These results suggest that the type and frequency of communication behaviour differs depending on the type of social organization in Equus species.