The life cycle of the broad tapeworm Diphyllobothrium latum is discussed. To complete its development from egg to adult worm the parasite has to pass through two obligatory intermediate hosts before invading its final host. At each stage critical ecological parameters have to be encountered, among them oxygen concentration, light intensity and temperature. The first intermediate host, which remained unknown for a long time, is one of many copepod species; the second hosts are fish, notably species belonging to the genera Esox. Perca and Lota. The parasite remains in the latter until eaten by Man and his domesticates. The prerequisites for the parasite's development are precise, and the elimination of only one is sufficient to free an area of infection.