Mass-scale production of Plasmodium vivax sporozoites in Anopheles stephensi was achieved using the chimpanzee (Pan troglodytes) as a source of infective blood. Membrane feeding was as successful as feeding mosquitoes directly on the animal so long as the time between drawing the blood and feeding was restricted to 45 min. Longer delays such as 2-3 h resulted in loss of infectivity in terms of oocyst production. The selected strain of A. stephensi was highly susceptible to P. vivax (Chesson strain). A strain of A. stephensi relatively refractory to P. falciparum showed no cross-refractoriness to P. vivax. Mixed infections of P. falciparum and P. vivax did not interfere with each other in their development in A. stephensi. A second normal blood meal to mosquitoes infected with P. vivax increased the yield of salivary gland sporozoites.