To investigate the possibility that the host fever response in malaria may affect parasite development, we studied the effect of temperature on Plasmodium falciparum in erythrocytic culture in vitro. Growth was markedly suppressed at 40 degrees C compared with 37 degrees C, due to disruption of the second half of the 48-h erythrocytic cycle. However, young intraerythrocytic parasites, which are highly exposed to fever during natural infection, appeared to develop normally at 40 degrees C. Because of the differential temperature sensitivity within the erythrocytic cycle, asynchronous cultures could be synchronized by transient elevations of temperature. Pronounced synchronization was observed when cultures were exposed to periodic elevations of temperature that simulated the 48-h fever cycle of tertian malaria. These findings indicate that malaria fever might act to promote parasite synchronization in vivo.