The transition of the malaria parasite in the blood of the human host from its asexual, and actively replicating lifestyle inside red cells, to the gametocyte, the non-replicating sexual stage that is transmitted to the mosquito represents a critical vulnerability of the parasite that is being increasingly investigated as a target for interventions to eliminate malaria. Available evidence indicates that a range of host- and parasite-specific factors stimulate this transition. These include a number of antimalarial drugs that are very effective against asexuals. In the course of a series of experimentally induced blood stage malaria studies in human volunteers, we identified by PCR persistent and relatively constant low level circulating Plasmodium falciparum DNA. Hypothesising that this was due to circulating gametocytes, we developed an RTPCR assay to test for the presence of the gametocyte-specific transcript pfs25. As well, we tested the infectivity of the blood of these volunteers to competent Anopheles stephensi mosquitos. Results of these experiments proving transmissible gametocytemia will be presented, applications of the results to the evaluation of transmission-blocking vaccineswill be discussed, as will the implications of these findings for malaria control .