A point survey of malaria in Karachi revealed a concentration of malaria cases in the periurban perimeter and marginal areas where immigrating groups, including Afghan refugees and Biharis from Bangladesh are concentrated. Populations of Anopheles stephensi, Karachi's main vector, were low, and were feeding chiefly on buffaloes which are also concentrated along the periurban perimeter. Maintenance of malaria transmission in periurban zones may depend on immigration of susceptibles, bovid hosts which help maintain zoophilic vector populations, and mosquito breeding sites in water tanks or buffalo hoofprints. The vector's switch to man-biting behaviour may occur either due to a 'spillover' effect from buffalo corrals to surrounding houses, or when buffaloes become less available for biting. Four types of chromosome inversions were found in the Karachi An. stephensi; as this species is not a vector in similar habitats in Lahore and Pondicherry where such inversions are not prevalent, it is likely that its status as a vector is related to chromosomal polymorphism influencing behaviour.