Malaria epidemiologic and entomologic studies were performed during both the high transmission and low transmission seasons to characterize the Plasmodium falciparum malaria transmission at a proposed malaria vaccine trial size in Irian Jaya, Indonesia. The study population consisted of two subsets native Irianese men with lifelong exposure to malaria and transmigrants who arrived from a nonmalarious area 2.5 years before the start of the study. All subjects received a radical cure for malaria and were then monitored weekly by blood film. Both P. falciparum malaria attack rates and incidence densities were calculated transmigrants had a significantly higher rate P0.003 than the Irianese during the low transmission season study 20 weeks long but not during the high transmission season study 12-weeks long. Lack of exposure-induced immunity left the transmigrants at a minimum 17-25 greater relative risk of becoming parasitemic compared with the Irianese during the low transmission season study.