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This paper examines the relationship between malaria transmission and migration in three northern Thai villages. Data and observations indicate that land-poor families forced into swidden farming have greater contact with the primary vectors in Thailand - which breed in small pools in forested areas and shady clearings on hilly scrub terrain. Once infected, migrants from an endemic locus can introduce the parasite into an area with no transmission but potent vectors, thus becoming the cause of explosive epidemics; equally, non-immunes carrying out agricultural activities in or across forest and border areas can themselves be subject to seasonal morbidity. In addition to agricultural activities on clearings near forested areas, clandestine forest activities and cross border traffic contributes to the high prevalence of malaria in Thai border villages. Illegal economic activities - logging, poaching, cattle and goods smuggling - interferes with vector suppression campaigns and prompt detection of infected cases, and ultimately increases human infection not only within the mobile population, but also within the passive population of villages to which the migrants return periodically. Control measures therefore need to take into account the economic pressures which determine a high degree of mobility, the ethnic diversity of the groups which depend on fringe activities for their economic welfare, and the difficult geography of the areas in which they live. As long as economic circumstances forcing human-vector contact receives inadequate attention, better alternatives to current vector control campaigns (which are not effective amongst migrants) are not tried and malaria transmission continues.