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The environment surrounding Kisumu in Kenya is described, emphasizing the importance of the Nyando valley as a highly malarious area as compared with the Nandi and Kisii hills where only epidemic malaria occurs. Available data on malaria transmission are summarized in detail. The varying seasonal importance of Anopheles gambiae as a vector in the different topographical areas is discussed. Data resulting from a brief survey of the A. gambiae complex (November 10th–December 3rd, 1970) conducted during very hot and dry weather are presented to show that Species A and Species B resting in houses are differentially distributed in this part of Nyanza. Species B predominated in the lowlands and was prolific in the artificially irrigated areas. Species A predominated in the highlands. Both sibling species were well represented in sympatric association among foothills. Only 3 females of A. gambiae (1A and 2B) were obtained from the experimental area of 23 square miles employed by the WHO for hut testing of fenitrothion for anopheline control. Circumstances indicated that both Species A and Species B would have formerly occured more abundantly in this zone. It is suggested that although Species A was found to be virtually restricted to the highlands and foothills at this season, populations of A would probably encroach into the lowlands during rainy periods. Finally, WHO data on parous rates of A. gambiae at 2 localities near Kisumu are presented to show that Species B may continue reproducing at a greater rate than Species A as the dry season progresses.
G. B. White (Sat,) studied this question.