The indirect immunofluorescence (IIF) test was used to detect persons with malaria antibody, with or without patent parasitemia, in a campaign to detect imported malaria and to prevent its introduction into the vector in the Sutter-Yuba area of California over the period 1975–1979. In addition to 176 slide-proven imported cases of Plasmodium vivax, 94 other persons reactive serologically but no patent parasitemia was found. Slide and serologic results indicated that the dominant malaria species involved was P. vivax. Seven patients who were both serologically and parasite negative when first tested subsequently reported with patent parasitemia and high antibody titer. One of these quiescent cases had resided in the United States at least 7 months prior to the first negative serology, and for some period thereafter before development of parasitemia and antibody response. Serologic results indicated that some persons had had contact with P. falciparum. When parasitemia is rapid in onset and serum is taken immediately, serologic response may be lacking. For detection of presumptive malaria infections, serology may have certain advantages not available with slide examinations. Blood slide examination and malaria serology are used to best advantage together, but either may be valuable in itself, alone, in efforts toward case detection and transmission control in hypoendemic malaria.