ABSTRACT. – The study of host-parasite associations has attracted much attention in the quest of unveiling the mechanisms behind the coevolutionary relationships of tightly associated organisms. We studied the cophylogeny of species of three genera of Monopisthocotylea, Lamellodiscus, Dactylogyrus and Gyrodactylus, and their fish hosts to determine which evolutionary events determine speciation and identify a common set of principles driving coevolution in the three host-monogenean systems studied. We used an event-based method (Jane) and a distance-base method (PACo) for cophylogenetic analysis. PACo revealed a significant coevolution signal in all three systems. In the significant coevolutionary scenarios retrieved by Jane, the number of cospeciations was always low, the number of failures to diverge was always high and relatively constant, and the number of losses was always the highest. The number of duplications and duplications with host-switching was more variable, but relatively high in most situations. These results suggest that speciation in the three host-monogenean systems is not driven primarily by the classical synchronic cospeciation process postulated for terrestrial host-parasite systems. Instead, pseudocospeciation (associated to host-switches), duplication and failure to diverge seemed to play a major role in shaping the current diversity and relationships to their hosts of the monogeneans studied. Therefore, evolutionary mechanisms leading to host specificity do not seem to primarily result from cospeciation.