Mosquitoes collected monthly for 1 year from human habitations in the Kisumu area of western Kenya were identified by morphological characters as Anopheles gambiae Giles sensu lato (An. gambiae s.l.) or An. funestus. Of the mosquitoes collected, 7,244 (67%) of the An. gambiae s.l. and 8,511 (87%) of the An. funestus were tested by enzyme-linked immunosorbent assay (ELISA) for the presence of Plasmodium falciparum circumsporozoite (CS) protein. ELISA positivity rates were 8.2% for An. gambiae s.l. and 6.1% for An. funestus. Both An. gambiae and An. arabiensis were detected among 432 ELISA-positive and 668 ELISA-negative An. gambiae s.l. identified to species with a ribosomal DNA probe. The species-specific infection rates were calculated to be 9.6% for An. gambiae and 0.4% for An. arabiensis. These results confirm that An. gambiae and An. funestus are the primary malaria vectors in western Kenya and that An. arabiensis is a relatively minor vector.