Among a group of 178 adult missionaries in Ethiopia, icteric hepatitis occurred at an annual rate of 5.5%. In another group of 57 adults in Ethiopia, and among 66 adults in Sudan, the annual rates were 3.9% and 2.0%, respectively. The incidence among children tended to be lower. The incidence of hepatitis continued at a high level for many years. As a prophylactic, γ-globulin would have been required for many years, not only for the first few. Not more than 55% appeared likely to develop immunity because of inapparent (anicteric) infection. The rate of second bouts of icteric hepatitis was about one fifth the rate for first attacks. The presence of a cold season in the climate may have contributed to the high incidence of hepatitis in Ethiopia, as compared to Sudan.