A DNA probe consisting of 21 base pair repeats obtained from a Tanzanian isolate of Plasmodium falciparum, cloned in pBR322 and labeled with 32P by nick translation was used to detect malaria parasitemia in samples obtained during a malaria survey undertaken in The Gambia. In an initial trial the hybridization assay had a specificity for P. falciparum of 100% and a sensitivity of 68%. False negative results were obtained only on samples with low parasitemia. Assay of red cells collected during an earlier malaria survey which had been stored for 1 year at -20 degrees C gave a higher level of sensitivity (85%), suggesting a beneficial effect from freezing and thawing. This was confirmed by examining in the same assay red cells processed immediately after collection and after 2 weeks of storage at -20 degrees C. Freezing and thawing gave a 21% increase in positivity, and a sensitivity of 100% was achieved with the frozen samples. Quantitation of autoradiographs by visual inspection and by scintillation counting gave a reasonable correlation with parasite counts. The DNA hybridization assay has considerable promise as an epidemiological tool.