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Entomological and parasitological investigations were carried out on malaria vectors and disease prevalence in two sets of villages, the highly irrigated Indira Gandhi Nahar Pariyojana (IGNP) command-area villages (Madassar and Awai), and the truly desertic non-command (unirrigated) area villages (Kanasar and Khetusar), located in different ecological conditions in the Thar Desert, north-western Rajasthan (India). Malaria prevalence, as determined through sustained fever surveys, was higher in the IGNP villages with a slide positivity rate (32%) marginally more than that of the unirrigated villages (25.5 %), but with a high proportion of Plasmodium falciparum (76.6%) in the former villages as compared to the latter (16.6%). Anopheles stephensi, A. culicifacies and A. subpictus were amongst the eight anopheline species collected from all the four villages which were found positive for malarial parasites. Anopheles stephensi was the predominant species in the unirrigated villages ( > 95%), although in the irrigated villages A. culicifacies was also found. The major ecological changes associated with irrigation in the Thar Desert are understood to be playing an important role in accentuating the transmission of malaria by improving vector breeding conditions and survival in an otherwise hostile arid environment. (C) 2001 Academic Press.