Seasonal occurrence in terms of prevalence, intensity of infection, abundance and density of the tapeworm Proteocephalus longicollis (Zeder, 1800) and its transmission between its intermediate host (Cyclops abyssorum prealpinus) and definitive host (common whitefish, Coregonus lavaretus) in Lake Annecy, an oligotrophic lake in the western part of the Alps, France, were studied in the period of 1998-2000. A copepod Cyclops abyssorum prealpinus (Kiefer, 1939), the dominant species among planktonic crustaceans in the lake, served as the only intermediate host for this parasite. Infection with plerocercoids was higher in adult copepods (predominantly females) than in copepodite stages IV and V. The prevalence rate of 25% found in C. abyssorum prealpinus females in June 1998 represents a unique infection rate of intermediate hosts with fish tapeworms in natural conditions. The final host, the common whitefish Coregonus lavaretus (L.), was heavily infected with P. longicollis throughout the year (prevalence 90%; mean abundance 40.3; maximum intensity of infection more than 500 tapeworms per fish); immature tapeworms predominated in all samples (P<0.01). Transmission of tapeworm larvae from copepods to the common whitefish took place most intensively in summer and autumn, and depended on seasonal changes in the density of the C. abyssorum prealpinus population, infection of this copepod with plerocercoids and their density in the lake. In addition, transmission efficiency also seems to be determined by the longevity of tapeworm larvae in the intermediate host, timing of predation of the fish host and rapid development of the parasite within this host during the summer period. Overall transmission potential of P. longicollis was low, with only about 9% of juvenile specimens reaching maturity in common whitefish.