It has often been suggested that vector–borne parasites alter their vector's feeding behaviour to increase their transmission, but these claims are often based on laboratory studies and lack rigorous testing in a natural situation. We show in this field study that the malaria parasite, Plasmodium falciparum, alters the blood–feeding behaviour of its mosquito vector, Anopheles gambiae s.l., in two ways. First, mosquitoes infected with sporozoites, the parasite stage that is transmitted from the mosquito to a human, took up larger blood meals than uninfected mosquitoes. Whereas 72% of the uninfected mosquitoes had obtained a full blood meal, 82% of the infected ones had engorged fully. Second, mosquitoes harbouring sporozoites were more likely to bite several people per night. Twenty–two per cent of the infected mosquitoes, but only 10% of the uninfected mosquitoes, contained blood from at least two people. We conclude that the observed changes in blood–feeding behaviour allow the parasite to spread more rapidly among human hosts, and thus confirm that the parasite manipulates the mosquito to increase its own transmission.