Procedures are described for mass cultivation of Schistosoma mansoni. Cercariae were pooled, concentrated, and axenized through a series of washes in base medium containing antibiotics. Cercariae were sheared through a double-Luer-ended needle connected to two syringes, and tails separated and discarded. Young schistosomules were grown in a medium based upon BME augmented with lactalbumin hydrolysate, glucose, hormones, and other amendments and supplemented with human serum. Human blood cells (Type O) were added to cultures. Intestinal pigment was seen on the 5th day and gut cecal fusion began on day 11. Schistosomules developed steadily to pairing, which was seen first during the 7th wk of culture. Pairing occurred somewhat later in relation to development in vivo.