The ability of malaria parasites to respond positively to the presence of feeding mosquito vectors would clearly be advantageous to transmission. In this study, Anopheles stephensi mosquitoes probed mice infected with the rodent malaria parasite, Plasmodium chabaudi. Growth of asexual stages was accelerated and gametocytes appeared 1-2 days earlier than in controls. This first study, to our knowledge, of the effects of mosquitoes on 'in-host' growth and development of Plasmodium has profound implications for malaria epidemiology, suggesting that individuals exposed to high mosquito numbers can contribute disproportionately high numbers of parasites to the transmission pool.