Administration of suspensions of Plasmodium berghei sporozoites by the oral route to groups of susceptible animals (A/J mice, young rats, hamsters, and Thamnomys) resulted in high rates of infection (66 to 100 percent). Control animals each given a suspension of 200,000 to 500,000 erythrocytes infected with P. berghei by the same oral route did not develop parasitemia. Direct intubation of sporozoite suspensions into the stomach failed to produce parasitemia. In vitro studies showed that sporozoites kept in medium M 199 acidified to pH 3.6 (the acid environment of the rat stomach) for 5 to 15 mtinutes lost their viability and infectivity. We believe that sporozoites of P. berghei find their way to the bloodstream during their brief sojourn in the esophagus.