Studies were made on living material supplemented by sections, of the development of the sporocyst stages of Lechriorchis primus. After hatching in the digestive tract of the snail, the miracidia penetrate the intestinal wall and form small sac-like mother sporocysts containing small numbers of germinal cells. The mother sporocysts which are attached to the outside of the intestinal wall grow into larger sacs containing greatly increased numbers of germinal cells. Soon they send out branches, chiefly into the digestive gland, and form extensively branched sacs. Next, ingrowths of the wall divide the mother sporocyst into compartments each containing groups of germinal cells. As the germinal cells start to grow into embryos, each becomes surrounded by cells of the mother sporocyst wall, which form a permanent outer layer, the paletot, around the developing daughters. A few migrating stages of daughter sporocysts like those described for Plagiorchis muris have been found in some infections, but usually the daughters go through the characteristic growth stages and complete their development in masses in the digestive gland of the snail without migrating from the mother. In early daughter sporocyst embryos the germinal cells undergo a very limited number of divisions before the germinal mass is formed. These produce a few precocious cercarial embryos which are quite well developed before new embryos begin to be produced by the germinal mass. A few almost fully developed cercariae from these early divisions of the germinal cells were present as early as 17 days after experimental infection.