Cowdria ruminantium was isolated from a natural infection of heartwater in a captive sitatunga (Tragelaphus spekei) which died after a short illness in Jos Zoo, Nigeria. The isolate killed laboratory mice in a shorter period of time than did isolates of C. ruminantium recovered from natural cases in cattle. This is the first confirmed case of heartwater in a captive wild ruminant in Nigeria, where the disease is endemic.