The pseudophyllidean cestode Ligula intestinalis (Lin. 1758) was studied in order to reach a fuller understanding of its biology and life cycle in a natural environment. The plerocercoid larva in rudd, Scardinius erythrophthalmus, was intensively studied by sampling the fish population of the reservoir. Environmental factors affecting the plerocercoid directly and also mediated through the host's physiology were studied in one generation of rudd sampled during a period of two years. The site of the plerocercoid in the fish was accurately determined by dissection and by sectioning of infected fish. Morphological changes during the development of the plerocercoid were investigated together with accompanying changes in nitrogen, glycogen and lipid content. Changes associated with senescence of the plerocercoid were observed from sections. Stunting of the rudd and changes in its behaviour were observed to result from infection with the plerocercoid. Final hosts in the life cycle were observed on the reservoir and the size of infected fish eaten by these birds* The adult phase of the life cycle was obtained in the laboratory by in vitro culture and, by experimental infection of ducks. Eggs obtained from in vivo culture in ducks were used to determine the effort of light on the hatching of the coracidia. Fish of various families harbour plorocercoids of Idatiajouleatialla and a hoot-lief was compiled showing the distribution and specie° of fish infected* Evolutionary development of the cestode in several fish families is discussed*