Sera from Somalis of both sexes between the ages of 16 and 60 were examined for leptospiral agglutinins. 37% of 105 apparently healthy individuals living in the arid Mogadishu area were positive, as were 64% of 107 schistosomiasis patients living in two villages on the Shabeele River (50.5% over-all). Pools of sera from similar subjects, as well as leprosy patients living on the Juba River and patients in Mogadishu hospitals with suspected viral hepatitis showed a similar prevalence rate of 56%. These figures are higher than prevelance rates for leptospiral antibodies generally found in other parts of the world, and in part may be related to the nomadic, cattle-driving existence common in Somalia. The titres of 11.2% of the positive sera examined singly indicated recent infection. Approximately twice as many subjects from the river villages as from the Mogadishu area were positive for more than one serovar, and a greater number of serovars were recorded from the villages. Antibodies to bratislava serovar, not previously recorded in Africa, were found in 57% of positive subjects, showing the highest prevalence rate among the investigated serovars. Co-antibodies to saprophytic Leptospira biflexa serovars were found in many of the sera.