SUMMARYThis paper presents information collected during a survey of stream‐dwelling triclads in two CO ntrasted areas of North Wales. The first study area was located in Caernarvon‐shire on the mainland of North Wales and included a mountainous region rising to over 1000 m with abundant cold springs. In contrast, the study area on the nearby island of Anglesey was characterized by relatively warm, slow‐flowing streams passing through gently undulating land which did not rise above 153 m. The object of the survey was to collect background data so that suitable streams could be chosen for more intensive studies on factors determining the distribution and abundance of stream‐dwelling triclads. Distribution maps for the commonly occurring species were prepared and observations on the relationship between stream source‐type and triclad fauna were obtained. The possible role of interspecific competition was of particular interest in this study and the results of this second phase of the work will be presented in a la