Abstract. – The great majority of studies on host-parasite associations, either theoretical or empirical, have focused on cospeciation and host-switch minimizing the importance of sympatric speciation, a mode of speciation that can be also defined as intra-host speciation. Sympatric speciation is considered when a single host species is infested by a monophyletic parasite lineage. The parasite speciates without a corresponding host cospeciation event and this leads to two or more lineages of parasites on single host species. A recent study illustrates how the diversification of Dactylogyrus spp. parasitizing cyprinid fish meets the four conditions required to recognize a case of sympatric speciation, according to Coyne (2007). However, inferring mode of speciation does not help to depict the mechanism of sympatric/intra-host speciation and theoretical model of speciation can be useful to investigate the conditions of sympatric speciation in parasites, as it has been already done for free-living organisms. Finally we aim at highlighting that diversification of parasites through intra-host speciation is maybe a prevalent mode of speciation in parasites, as the numerous number of parasite species flocks suggest, and that parasites are also good models for investigating the mechanisms of sympatric speciation.