Domestic rabbits were exposed in the Pocomoke Cypress Swamp from early May through October 1973, to act as sentinel indicators of Keystone (KEY) virus transmission on the Delaware-Maryland-Virginia (DelMarVa) Peninsula. Rabbits were bled weekly and their sera were tested for the presence of neutralizing antibody to both KEY and Jamestown Canyon (JC) viruses. Adult mosquito activity was monitored concurrently by CDC miniature light traps with Dry Ice. Of 100 rabbits exposed, 20 rabbits acquired neutralizing antibody to KEY virus as a result of their exposure; none acquired antibody to JC virus. Eighty-three percent of the susceptible rabbits exposed converted immediately after the emergence of Aedes atlanticus adult mosquitoes in early September. Rabbit seroconversions were not limited to any particular habitat within the swamp. The conversion of a large proportion of sentinel rabbits immediately following emergence of A. atlanticus, and the previous demonstration of transovarial transmission of KEY virus by A. atlanticus, suggest that vertically infected individuals are capable of viral transmission at their initial engorgement. Such transmission provides the initial step in a vertebrate amplification cycle.