A malaria study was carried out in an oil exploration concession in northwestern Peru on the Rio Pastaza. Stained blood slides were examined and a serological test (indirect immunofluorescence) carried out. Oil field workers and native Jivaro Indians were tested. Two matched age groups had comparable percentages of malaria as determined with stained blood slides. However, serological tests indicated that the native Indians had had far more experience with malaria. They also reflect some success in efforts to protect the workers from malaria. The focus appears to be caused entirely by Plasmodium vivax, with P. falciparum entirely absent and P. malarial probably so. Malaria control measures should be instituted among the native population for their own benefit as well as to further protect the workers. © 1981 Liverpool School of Tropical Medicine.