Signal crayfish (Pacifastacus leniusculus) is an alien species spreading planned and unplanned in Finnish freshwaters. Signal crayfish has been introduced to compensate for the reduction of endemic noble crayfish stocks (Astacus astacus) due to crayfish plague. Signal crayfish has been introduced also into areas with no previous crayfish population. Compared to the endemic species, signal crayfish is more resistant to the plague and has faster growth and reproduction rate. Adult signal crayfish is larger in size than the endemic species and more aggressive in its behavior. As an alien species, it can change the structure and function of the ecosystem. Crayfish feeds partly on freshwater invertebrates. These same invertebrates can act as hosts for fish parasites. By eating invertebrates, signal crayfish might affect the abundance and prevalence of fish parasites. In this study, the aim was to investigate if the signal crayfish can affect the parasites of roach (Rutilus rutilus) in a large lake ecosystem, previously naive to crayfish. The study was conducted at lake Päijänne, the parasites were determined from roaches (n = 188) caught in the study sites (n = 10) by gillnets. The average abundances and prevalences of parasite species found from roach in each study area were compared between areas with (n = 5) and without (n = 5) signal crayfish. Comparisons were made also by grouping the species according to their invertebrate hosts. The study revealed less Acanthocephalus lucii –infections on roaches caught from study sites with signal crayfish. The intermediate host of A. lucii is waterlouse (Asellus aquaticus). Rhipidocotyle fennica, a trematode parasitizing bivalves, was more prevalent on roaches caught in areas with crayfish. There were no significant differences when comparing parasites as groups infecting bivalves, gastropods or copepods. This study brings out new information about the effects of signal crayfish, an introduced alien species, on fish parasite communities. On the other hand, it has been demonstrated that the occurrence and strength of effects depend also on the ecology and habitat of fish hosts.