Hepatitis B surface antigen (HBsAg)-positive blood meals were fed to a colony of the common bedbug (Cimex lectularius L) in a series of 5 experiments. Antigen persisted in the bugs for at least 7 1/2 weeks, but was undetectable after 18 weeks. Trans-stadial transmission was demonstrated through one moult only, and transovarial transmission did not occur. Antigen was successfully transmitted by adult bugs into 3 out of 35 canisters of HBsAg-negative blood. Antibody of HBsAg was detected in the serum from a rabbit on which HBsAg-positive adult bugs had fed as well as in the serum of 2 out of 10 guinea-pigs on which HBsAg-positive 4th and 5th nymphal instars had fed. The results as a whole indicate that biological multiplication and biological transmission do not occur in C. lectularius, but mechanical transmission has been demonstrated. This is probably an important means of hepatitis B virus transmission among humans in South Africa.