In May 1992, two Gyrodactylus species were found on wild Atlantic wolffish, Anarhichas lupus L., caught off northern Norway. Both species belonged to the G perlucidus-group Malmberg, 1970. In one species, the taxonomically important opisthaptoral hard parts were significantly different from previously described species. The other species was conspecific with or similar to G. errabundus Malmberg, 1970. In October 1992, this latter species caused gydrodactylosis and mortality among juvenile and adult Atlantic wolffish and spotted wolffish, A. minor Olafsen, in a fish farm in northern Norway. Treatment against the gyrodactylosis by immersion in a fomaldehyde bath (1:4000) was successful.