A sero-epidemiological study of malaria, with special emphasis on Plasmodium brasilianum/P. malariae, was conducted on 4 Indian tribes living in the Amazon Basin of northern Brazil: the Arara, the Parakana, the Asurini, and the Metuktire. The incidence of malaria, as determined by blood films, was very low in all tribes. Parasitemia levels in most individuals were less than 0.02%; determination of the plasmodial species was not feasible. High levels of antibodies to both blood stages and sporozoites were detected for P. brasilianum/P. malariae, P. falciparum and P. vivax. The anti-sporozoite antibody response against all 3 plasmodial species was age related. All of the Metuktire adults and almost 90% of the Asurini adults had anti-sporozoite antibodies against P. brasilianum/P. malariae. The presence of P. brasilianum was confirmed in many of the indigenous monkeys by blood films and serology. This suggests that the monkeys, which are often kept as pets, serve as reservoir hosts. Anopheles darlingi mosquitoes, infected with P. brasilianum/P. malariae, were found in the study area.