The fate of hepatitis B virus in the bedbug was investigated to assess this insect's potential as a vector. Colony-reared Cimex hemipterus (Fabr.) were fed once on blood positive for hepatitis B surface antigen (HBsAg). The insects were sampled at intervals thereafter and tested for HBsAg by radioimmunoassay. HBsAg persisted for up to six weeks in the bedbug's body after a single HBsAg-positive meal, during which time several further HBsAg-negative blood meals were taken. This result explains the high rates of field infection in bedbugs and further supports the hypothesis that bedbugs may play a role in transmission of hepatitis B virus.