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The Thar Desert region in north-western India is traditionally understood to be only mildly prone to malaria. During the last six decades, a major part of this arid zone has undergone vast physiographic and climatic changes, resulting in the formation of extensive mosquito breeding habitats of varying nature, high vector densities and intrusion of new malaria mosquitoes earlier unknown for the region. As a result, the prevalence of disease in the desert, with several epidemics reported in the last 10 years, has changed a great deal for the worse. It is evident from data for the period 1961-1994 that not only has the slide positivity rate in the desert region increased 3.5-fold, but Plasmodium falciparum incidence also increased as much. Along with Anopheles stephensi, the traditional malaria vector in the xeric environment, another significant vector of the Indian mainland, An. culicifacies, has also established itself in the areas extensively irrigated through canals. A review of the malaria situation in the State, particularly following extensive canalization in north-western desert areas, indicates the formation of new disease foci in the hitherto less malarious Thar Desert. (C) 1997 Academic Press Limited.