The measure of order and disorder in the distribution of species in fragmented habitats proposed by Atmar and Patterson (Oecologia, 96 (1993) 373-82) was applied to investigate nested patterns of Dactylogyrus species parasitising the gills of roach. Organisation in dactylogyrid assemblages was investigated at three levels: (1) host populations between localities; (2) local host populations over seasons; and (3) individual hosts over one season within a local host population. Dactylogyrid assemblages showed nested patterns when analyses were conducted at the level of localities (among host populations) and at the level of seasons (among host populations within localities). The analysis at the level of hosts (infracommunities of parasites) revealed that nested pattern is not common. We suggest that nestedness may have a variety of causes and does not necessarily imply competition.