Culex tarsalis and Aedes aegypti mosquitoes were fed on chimpanzees carrying hepatitis B surface antigen (HBS Ag) of known infectivity and pools were tested by radioimmunoassay daily for the presence of HBS Ag. HBS Ag continued to be detected at low levels in mosquito tissue after digestion of the blood meal. Inoculation of susceptible chimpanzees with macerated pools of A. aegypti mosquitoes at two intervals after digestion of the blood meal did not produce hepatitis or serologic evidence of hepatitis B virus infection. Mechanical transmission studies by interrupting feeding of A aegypti from HBS Ag-carrier chimpanzees and transferring them to susceptible chimpanzees did not produce hepatitis. These findings do not support the hypothesis that mosquitoes are involved in either biological or mechanical transmission of hepatitis B.