Two subcellular fractions from the midgut of the malaria mosquito Anopheles stephensi (Liston) were used to immunize BALB/c mice. Mice were subsequently infected with the rodent malaria parasite Plasmodium berghei (Vincke & Lips), and the effects of anti-mosquito immunity on mosquito survival and fecundity and on parasite transmission were investigated. Mosquitoes were infected directly from mice (in vivo) or by feeding cultured ookinetes through a membrane (in vitro). Infections were monitored by counting oocysts on the midgut wall. Microvilli extracts induced a strong and partially specific antibody reaction against the midgut, which was manifest as decreased survival in in vivo fed mosquitoes and reduced fecundity in both kinds of feeding. Antisera against microvilli reduced the mean intensity of P. berghei oocysts when fed in vitro, while mosquitoes fed antiserum against basolateral plasma membranes in vivo, showed higher oocyst burdens.