The viremic period in arbovirus infections of primates is frequently accompanied by a rise in body temperature. It has been suggested that this response of the vertebrate host may be advantageous to the parasite, since the raised temperature may enhance the attractiveness of the host to feeding mosquitoes. The viremia is usually transient, lasting for 2-4 days only and when followed by life-long immunity, it is the only period in the whole life-span of the primate during which virus is available for transmission. This hypothesis is now supported by the results of laboratory tests, in which mosquitoes were allowed the choice of alternative artificial warm sources, one of them 1-3 C warmer than the other. Provided that both sources were present simultaneously the mosquitoes selected the warmer of the two in significantly greater numbers.