Summary and Conclusions In a Panama strain of Plasmodium falciparum 23 inadequately treated infections persisted for an average of 279.5 ± 19.9 days, with a range in duration from 114 to 503 days. Four cases persisted for more than a year. The initial period of continuous parasitemias in 37 cases persisted for an average of 115.7 ± 7.5 days. This was followed in 23 cases by a period of terminal intermittent parasitemias with a mean duration of 168 ± 19.0 days. There was little difference between blood and sporozoite induced infections where duration of the infection was concerned. The results reported extend to a tropical strain the findings of Eyles and Young (1951) on the duration of a temperate zone strain of P. falciparum. The application of these findings to certain field conditions is discussed. The Panama strain of falciparum apparently is somewhat more virulent than the United States strain. Although the maximum parasite densities were similar in the two strains, the Panama strain was nearly double the U. S. strain in the length of initial clinical episode, number of clinical episodes and total hours of fever. Other criteria suggestive of strain difference are the significantly longer duration of terminal intermittent parasitemias in the Panama strain, the absence of cross-immunity between the two strains, and differences in gametocyte morphology.