Antibody responses to malarial antigens were determined in 614 serum samples collected from the Wopkaimin population of the Star Mountains of Papua New Guinea. In point prevalence surveys made in 1982–1983, 33.7% of the persons examined were infected with Plasmodium falciparum, P. vivax, or P. malariae. Of these, 72.9% were infected with P. falciparum. In a standard fluorescent antibody test, highest level responses were to P. falciparum, followed by P. malariae, P. vivax, and P. ovale. A strong correlation was found between results of the fluorescent antibody tests and those obtained in an enzymelinked immunosorbent assay using P. falciparum antigens. The failure of immune responses to eliminate these species of Plasmodium in this highly isolated population is discussed.