Using the Trager-Jensen method, a second malaria species. Plasmodium fragile, a simian counterpart to the human malaria P. falciparum, has been cultivated successfully. The average growth rate every 3–4 days was 5-fold and the average number of merozoites observed was 14. To date, only rhesus monkey red blood cells (RBC) would support the long-term cultivation of this parasite. Short-term observations indicate that RBC from the squirrel monkey (Saimiri sciureus) may support growth but human RBC of each of the four major AB-O types failed to support growth of the parasite. Availability of the P. fragile-rhesus monkey model would allow for a second parasite-host system for the in vitro and in vivo study of the immunologic responses of the falciparum-like parasite in a more natural host.