Coinciding with a population decline in the invasive spiny-cheek crayfish Orconectes limosus in Lake Constance, SW Germany, we found crayfish specimens with a fungus-like Aufwuchs which after DNA-isolation and sequencing was identified as consisting of a set of previously undescribed Saprolegnia species. This finding may have implications for the farming and conservation of native crayfish as well as for the lake's ecosystem. We propose that spiny-cheek crayfish might function as a disease vector for these potential pathogens. © 2008 E. Schweizerbart'sche Verlagsbuchhandlung.