Since in humans, skin temperature is lower than internal temperature, the temperature sensitivity of Leishmania may influence the tropism of Leishmania in the human host; temperature-sensitive parasites may remain in the skin, temperature-resistant parasites may go to the viscera. In order to pursue the genetic factors controlling Leishmania tropism, we have developed an in vitro promastigote temperature model. Promastigote growth is measured at 30, 32, and 34 degrees C and compared with growth at the control temperature (25 degrees C). The results from tests of the promastigote temperature sensitivity of eight species (33 different strains) show that visceral species (L. donovani and L. chagasi) are more temperature resistant than cutaneous species (L. major, L. tropica, L. mexicana, L. braziliensis, L. panamensis, and L. amazonensis), that Old World species are more temperature-resistant than New World species, and that within the New World cutaneous species there are three distinct temperature sensitivity groupings (L. mexicana > L. braziliensis and L. panamensis > L. amazonensis). Interestingly, viscerotropic L. tropica from Operation Desert Storm and L. donovani complex strains isolated from cutaneous lesions are more and less temperature-sensitive, respectively, than strains of the same species with the expected tropism in vivo.