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Anopheles arabiensis and A. gambiae are the two main members of the A, gambiae complex inolved in the transmission of malaria in Madagascar. In the central highlands, above 1000 m, A. arabiensis is the only species present. Elsewhere in the country the two species are sympatric and their frequencies appear to form a gradient belated to altitude and climatic factors; A. gambiae is more common with lower altitude and increased humidity. Comparison of the different caryotypes of the polytenic chromosomes in the ovaries of each species shows that: 1) A. arabiensis is characterized by Xbcd. 2Rb/+, 2La and 3Ra/+, with only 2Rb/b homozygous individuals present in the central highlands, and 2Rb/+ polymorphic populations only occur at low altitudes on the dry western plains and the semiarid southern plateau; 2) A. gambiae is characterized by Xag, 2Rb/+, and 2La/+, with 2Rb/+ polymorphic chromosomes present only in the semi-arid southern plateau, and 2Rb found in all other areas where the species is present. Comparison with continental Africa confirms that populations of A. gambiae and A. arabiensis in Madagascar are very close if not identical to those in East Africa (Ethiopia, Mozambique, Tanzania, Kenya, and the Comoro Islands) and Central Africa (Burundi), but differ from those in West Africa (Gambia, Guinea-Bissau, Burkina Faso, Senegal, Togo and Benin), in which numerous, complex chromosomal inversions occur.