Hepatitis A is a disease commonly found in Thai children. However, there are very few reports on the age specific prevalence in Thailand. We studied the hapatitis A virus (HAV) antibody titer in a population in the rural eastern part of Thailand, using an antiHAV ELISA test (Abbott Laboratories, North Chicago Ill). Three hundred and sixty four subjects from Pong Nam Ron, Chanthaburi Province and 236 of children and adults from Bo Thong, Chon Buri Province were studied for age specific prevalence of antiHAV. The immunity against HAV increased with age. Fifty percent of Pong Nam Ron children had antiHAV antibodies at the age of 8-9 years and at the age of 12-13 years of Bo Thong children. The overall antibody prevalence rate was 67.9% in Pong Nam Ron and 59.3% in Bo Thong Districts. According to our data, less than 30 percent of children under 10 years old in the eastern part of Thailand were seropositive for HAV. This finding indicated a much lower incidence than previously reported which may reflect better personal and food hygiene.