Transmission of malaria depends on the successful development of the sexual stages of the parasite within the midgut of the mosquito vector. The differentiation process leading to the production of the sexual stages is delineated by several developmental switches. Arresting the progression through this sexual differentiation pathway would effectively block the spread of the disease. The successful development of such transmission- blocking agents is hampered by the lack of a detailed understanding of the program of gene expression that governs sexual differentiation of the parasite. Here we describe the isolation and functional characterization of the Plasmodium falciparum pfs16 and pfs25 promoters, whose activation marks the developmental switches executed during the sexual differentiation process. We have studied the differential activation of the pfs16 and pfs25 promoters during intraerythrocytic development by transfection of P. falciparum and during gametogenesis and early sporogonic development by transfection of the related malarial parasite P. gallinaceum. Our data indicate that the promoter of the pfs16 gene is activated at the onset of gametocytogenesis, while the activity of the pfs25 promoter is induced following the transition to the mosquito vector. Both promoters have unusual DNA compositions and are extremely A/T rich. We have identified the regions in the pfs16 and pfs25 promoters that are essential for high transcriptional activity. Furthermore, we have identified a DNA, binding protein, termed PAF- 1, which activates pfs25 transcription in the mosquito midgut. The data presented here shed the first light on the details of processes of gene regulation in the important human pathogen P. falciparum.