Development of the mother and daughter sporocysts of a plagiorchioid trematode from snakes, Lechriorchis primus, was studied in both experimental and natural infections in Physa gyrina and P. parkeri. Most of the observations were made on living material, but sections were made of critical stages. The miracidium hatches from the egg in the digestive tract of the snail and penetrates the intestinal wall. The mother sporocyst usually develops attached to the outside of the wall of the first part of the snail’s intestine. In early stages it is a small round or oval sac containing four germinal cells. It increases rapidly in size and soon sends out branches along the intestine and mesenteries, and between the lobes of the digestive gland. The germinal cells multiply very rapidly and during the early stages completely fill the mother sporocyst, including its smallest branches. Soon the wall of the mother sporocyst grows in to form compartments each filled with germinal cells. As the mother sporocyst grows and its germinal cells develop into daughter sporocyst embryos, the mother sporocyst cells multiply enormously and form a matrix completely surrounding the germinal elements. Later each daughter sporocyst becomes surrounded with a layer of mother sporocyst cells forming an outer layer, the paletot. In the young daughter sporocyst embryos there is a limited division of the germinal cells to form cercarial embryos and a germinal mass. This serves as a center of multiplication of geminal cells to produce a constant supply of new cercarial embryos throughout the reproduction life of the daughter sporocysts. Part of the young daughter sporocysts migrate from the mother and invade other parts of the digestive gland of the snail where they go through the characteristic growth stages. Others go through the growth stages and complete their development without leaving the mother. The first cercariae mature about 20 days after infection.