In an endemic area of Costa Rica nonparenteral type B hepatitis occurs at a yearly rate of 160/100,000, in an endemoepidemic pattern with periodic localized outbreaks. In a recent episode in the village of San Rafael, 23 clinical and 67 subclinical cases, all subtype adw, originated from two carriers living in opposite ends of the village. Contact transmission was identified as the mode of spread.In a coincidental outbreak in another village, San Juan, 17 clinical and 40 subclinical cases were observed. Thirty-seven cases of HBSAg/ayw positive hepatitis occurred in the Western sector of the village, apparently originating from contact with three HBSAg/ayw carriers found in that area. There were also 20 cases of HBSAg/adw positive hepatitis, but these occurred almost exclusively in the Eastern sector, where subtype adw disease has been prevalent in previous years. The geographic distribution of antibody subtypes found in the two separated sectors of the village coincided fully with that of the antigen subtypes. These observations show that personal contact is definitely a mode of transmission of hepatitis B.