1. A phylogenetic hypothesis for the triclads is presented and the characters on which it is based are discussed. 2. The sister group of the Tricladida is formed by the Bothrioplanida, and together the two taxa share a sistergroup relationship with the Proseriata. 3. The monophyletic status of the suborder Tricladida is supported by several derived features (Fig. 1., characters 9—12), including a unique embryological development and the presence of a marginal adhesive zone. 4. It is postulated that the Maricola is the primitive sister group of the Terricola and the Paludicola together and that the Paludicola represents the most advanced group within the Tricladida. Characters supporting these hypotheses are discussed (Fig. 1, characters 14-16, and 20-22, respectively). 5. Within the Paludicola the Planariidae and the Dendrocoelidae together are the sister group of the Dugesiidae (Fig. 1, character 24). 6. The monophyletic status of the Maricola and the Terricola is supported by one and three apomorphic characters, respectively (Fig. 1, characters 13, and 17—19). 7. Cases of conflicting or not sufficiently resolved character distributions are discussed and are evaluated in order to arrive at a phylogenetic tree providing the best fit to the data set considered.