An epidemiological study of X-cell gill disease in common dab Limanda Limanda L. was conducted at a series of stations in the eastern North Sea, the Skagerrak and the Kattegat during the period 1986 to 1993. At most of the stations the prevalence of X-cell gill disease varied from 0 to 1% throughout the whole period of investigation, except at 3 stations where high prevalences, up to 38%, were observed. Spatial and temporal variations of prevalence were observed. On one occasion it was possible to separate a highly affected dab population from a significantly less affected population within a distance of 500 m. This significant spatial variation was not found for the diseases lymphocystis, epidermal papilloma and skin ulcers. At one station the relative risk of contracting X-cell gill disease was significantly higher in female dab compared to males while the opposite was the case at the 2 other stations. X-cell gill disease mainly affected 2 and 3 yr old dab and had a serious effect on the condition of the fish. The condition factor of affected fish was significantly lower compared to healthy specimens. It was not possible to prove an increased mortality in infected fish, although their general condition was extremely poor.