1.The morphology of Protopolystoma xenopi from Uganda is compared with specimens previously described by Williams. The chief differences are the slightly greater maximum size, the presence of a second pair of anchors on the opisthaptor, and the absence of a second large oviducal chamber in the uterus.

2. Encapsulation of the egg is described from living worms. A thin, plastic membrane forms around the ovum and yolk cells within the ootype before all the yolk cells have passed into the ootype. The remaining yolk cells pass from the ovo-vitelline duct into the sac formed by this membrane, which is attached to the distal walls of the ootype.

3. Eggs are produced at the rate of 4–15 eggs/worm/24 hr. They hatch in 24–27 days at 24–26° C. The opisthaptor of the larva bears sixteen hooklets and two pairs of anchors, which are in the same positions as in the adult.

4. One larva with one pair of suckers was obtained from the alimentary canal or kidney of a metamorphosing Xenopus tadpole. About sixty other tadpoles were examined for P. xenopi but none was found.

5. Twelve P. xenopi were recovered from the kidneys of a small frog that had been infected experimentally 10 days previously.

6. When naturally infected small frogs from Kajansi Fish Farm were examined, immature P. xenopi were found in the kidney of one frog, in the urinary ducts of two frogs, and in the bladder of one frog. The infection rate was much lower than that in mature Xenopus. Immature P. xenopi were also found in the kidneys of mature Xenopus mülleri.

7. P. xenopi matures more rapidly than its host. A frog weighing only 2·9 g. contained a nearly mature worm in the bladder, while one weighing 4·2 g. contained two mature worms, shedding eggs. Eggs are shed throughout the year. The life cycle of P. xenopi is therefore not regulated by that of its host.