Processes connected with host specificity were investigated in congeneric monogenean parasites including 51 Dactylogyrus species parasitizing 19 fish species belonging to Cyprinidae and one fish species belonging to Percidae. Phylogenetic information obtained from molecular analysis of 18S rDNA and ITS1 of Dactylogyrus and cytochrome b of cyprinid fish were included in the analyses. We tested whether host specificity is constrained by parasite and host phylogeny. Morphometric correlates of host specificity i.e. parasite body size and variables connected with attachment organs were studied. Determinants of host specificity were investigated following the hypothesis of specialisation on more predictable resources such maximal host body size, maximal longevity and abundance. Host range was calculated for each parasite species. Moreover, index of host specificity was defined for this study including host relatedness. Host specificity was constrained by parasite phylogeny, i.e. it was related to intra-host speciation of Dactylogyrus species. No relationship between host specificity and fish host phylogeny was found. Specialist trait was showed as an ancestral state for Dactylogyrus species. We confirmed the specialisation on more predictable resource, i.e. the specialist parasites with larger body size colonized the fish with higher maximal longevity. Specialists with larger anchors (a part of attachment organs) tend to live on longer lived and/or larger body sized fish species. No morphometric association between the traits of preferred host and generalist parasites was found (Dactylogyrus generalists parasitize one preferred and several supplementary hosts). Evolution of morphology of attachment organ is connected with host specificity and fish relatedness. However, it seems that even generalists can generate morphological adaptation to their hosts.