Abstract. During 1981‐1986, the mean prevalence of Thelohania contejeani parasitism of the noble crayfish, Astacus astacus L., in Lake Steinsfjorden, S. E. Norway, was 0.29%. The crayfish population is heavily exploited. Thelohania contejeani infection levels increased significantly during the fishing season, and variations between years were more pronounced in catches after the fishing season (0.38‐2.11%) compared to catches before the fishing season (0.05‐0‐19%). Sixty‐eight per cent of the variation between years could be explained from variations in trap effort. Scuba‐diving yielded significantly higher T. contejeani infection levels than simultaneous trapping with baited traps. Thus, in years with heavy exploitation, a larger fraction of non‐infected crayfish are removed from the population as these are more susceptible to capture. The findings do not support the hypothesis that density and cannibalism influence T. contejeani infection. Copyright © 1990, Wiley Blackwell. All rights reserved
