A strain of Plasmodium falciparum, transmitted in Beaufort District, Sabah, near the town Membakut, was isolated in July 1972 and transferred to the University of Maryland for characterization in non immune volunteers. At Maryland the Sabah (Kal.) strain, as it was designated, was transmitted efficiently by colonized Anopheles stephensi. Prophylactically, it was not suppressed by chloroquine or amodiaquine 0.3 g (base) or pyrimethamine 25 mg taken by volunteers at weekly intervals; however, proguanil 200 mg daily (but not 100 mg) was effective. Attacks of malaria attributable to this strain were radically cured by quinine 8.1 g (base) given during 8 days (10.7 g of quinine sulphate), but parasitaemia recrudesced after 3 and 5 day courses of chloroquine and after one of 3 courses of amodiaquine. Proguanil and pyrimethamine were ineffective curatively, and the latter drug did not inhibit oocyst development in the mosquito, whereas primaquine was an effective sporontocide and gametocytocide.