1. 1. The incidence and intensity of infection of a species of Diphyllobothrium infecting powan ( Coregonus clupeoides) in Loch Lomond was estimated from a survey of 800 fish. 2. 2. Various laboratory animals, both avian and mammalian, have been found to act as definitive hosts; of these, the most convenient for experimental work was the albino rat. 3. 3. A method of infecting rats with plerocercoids of known weights has been described, and the rate of growth from the time of infection to maturity determined. 4. 4. Histological examination of whole mounts and sections showed that each stage of maturation occurred after specific time intervals in the gut of the definitive host. 5. 5. The distribution of worms recovered at autopsy, showed that Diphyllobothrium first established itself far back in the small intestine, but did not grow in this region. Normally a forward migration occurred after two days which coincided with the onset of the exponential growth phase, during which the worm doubled its weight every 24 hours. 6. 6. The growth rate decreased sharply on the 7th day when eggs were first produced. 7. 7. The probable causes of the different phases of development have been discussed and a possible hormone control mechanism suggested.