Plasmodium gonderi, one of the vivax-type simian parasites, has been cultured in vitro by the candle jar method and by the sealed flask method. It showed a 48-hour asexual cycle and maintained its morphologic integrity. The effect of different sera (human, rabbit, and horse) was tested in culture and compared with results using rhesus serum. Growth was lower in rabbit and horse serum, but higher in human serum. Compared with individual human and rhesus serum, the multiplication rate in combined rhesus and human sera (1:1) was greater. In addition, human erythrocytes proved refractory to invasion by P. gonderi in vitro.