To aid in the understanding of the problems which were encountered in the cultivation in vitro of Nippostrongylus muris, a brief description will be given of its normal cycle of development in nature. The male and female adult worms, approximately 4 to 6 mm. in length, reside in the small intestine of the rat. Eggs are passed in the feces, and the larvae which hatch in the soil feed primarily on living bacteria. Under appropriate conditions of oxygen and moisture, they moult twice and develop to infective third-stage larvae. These larvae penetrate the skin of a rat, migrate to the lungs where they grow and undergo the third moult. The newly emerged fourth-stage worms then migrate up the trachea and down the digestive tract to the small intestine where they feed on mucosal tissue and blood. Here they go through the fourth moult and form adult worms to complete the cycle.