The amphilinideans and gyrocotylideans are small groups of monozoic platyhelminths parasitizing fishes and occasionally turtles. They have traditionally been placed with the cestodes because they lack a gut. Systematic relationships of the two groups are poorly understood. Eight species of amphilinideans are considered valid. A phylogenetic analysis of 46 character states comprising 34 homologous series produced a single phylogenetic tree with a consistency index value of 89%, indicating a very low degree of homoplasy. The number of genera has been reduced from six to three, in an attempt to construct a classification which reflects the phylogenetic relationships of the group. The phylogenetic relationships exhibit a fairly high degree of congruence (70%) with a currently accepted hypothesis of phylogenetic relationship of the teleosts. This implies that coevolution has occurred, although with many parasite losses and extinctions. An analysis of the biogeographic distribution of the amphilinideans using the methods of vicariance biogeography produced complete or nearly complete congruence of the distribution of amphilinideans with hypotheses of area relationship, indicating that vicariance is the best explanation for the biogeographic distribution of the group. Ten species of gyrocotylideans are recognized, of which seven were examined in this study. A phylogenetic analysis of 24 character states comprising 13 homologous series produced two trees of equal length. The consistency index for each tree is 88%, indicating a very low degree of homoplasy. The phylogenetic analysis of the gyrocotylideans has provided a summary of the current knowledge of the gyrocotylideans which can serve as a framework for further investigations. Slightly more than half of the observed associations between gyrocotylideans and holocephalans can be attributed to coevolution. Remaining associations can be attributed to colonization, but these represent recolonizations of plesiomorphic hosts. The geographic distribution of gyrocotylideans remains enigmatic, as it does not appear to reflect plate movements. It is suggested that their distribution may be pfe-Pangaean in origin. The phylogenetic analyses of the amphilinideans and gyrocotylideans have provided corroboration for a hypothesis of relationship for the Subphylum Cercomeria. Seven new characters have been identified as autapomorphies for the amphilinideans and gyrocotylideans, while an additional seven new characters support the Subclass Cestodaria and the Infrasubclass Cestoidea. Amphilinideans and gyrocotylideans both appear to be relict groups, but of different types. The amphilinideans were probably more widespread and more numerous than they are at present (i.e. they are geographic and numerical relicts). The gyrocotylideans, on the other hand, appear to have never been diverse or widespread, and are therefore phylogenetic relicts. / Science, Faculty of / Zoology, Department of / Graduate