Serum samples collected monthly over a 34-month period from cattle, sheep and goats in the Greater Accra Region of Ghana were tested for antibodies to Ehrlichia (previously Cowdria) ruminantium, the causative agent of heartwater, by polyclonal competitive ELISA (PC-ELISA). Maternal antibodies, detected in about half of animals followed from under 1 month old, declined to negative levels within 2-4 months. Amblyomma variegatum tick vectors were present on livestock in rural areas throughout the year, and first seroconversion occurred at any age, although the majority of calves seroconverted between 1 and 10 months old, sheep by 11 months, and goats by 7 months. All the cattle in the study became seropositive by 20 months of age, except one animal which subsequently died of heartwater. Following seroconversion, 25% of bovine sera tested negative in the PC-ELISA. Just over half the sheep in the survey seroconverted before or during the study period; following seroconversion, less than 3% of ovine sera became PC-ELISA negative. About a quarter of the goats seroconverted, and 34% of their post-seroconversion sera tested negative in the PC-ELISA. Overall, the serology indicated that virtually all cattle on the survey farms were exposed to E. ruminantium without suffering disease, but that a substantial proportion of sheep and goats escaped exposure and thus formed a susceptible population. E. ruminantium was detected in brains of 14, 36 and 4% of cattle, sheep and goats submitted for post mortem at the Accra Veterinary Laboratory, indicating that sheep were most at risk from heartwater disease.