The FCR-3/FMG African strain of Plasmodium falciparum was cloned by the limiting dilution technique in in vitro culture to obtain parasite populations derived from a single cell. The basic in vitro culture technique used was that of Trager and Jensen, performed in microtiter culture plates. The cloning sequence was repeated serially three times, and three parasite clones with a higher than 99% probability of single cell derivation were isolated. Two of these clones were determined by electron microscopic examination to possess the K+ and one the K- trait. The clones were found to be equally sensitive to chloroquine in vitro but varied in their in vivo pathogenicity for Aotus, the K- clone being non-pathogenic.