Two allopatric populations of Polycelis nigra (MuUller), one from lowland productive Llyn Maelog on Anglesey and the other from upland unproductive Llyn Dinas in Snowdonia, were each subjected to competition in the laboratory from the same Polycelis tenuis Ijima population from productive Llyn Llygeirian, also on Anglesey, where it coexists with P. nigra. The triclads were fed weekly on crushed Asellus (Isopoda) and their biomasses calculated monthly over five months. Results suggest that sympatric P. nigra are more successful competitors against P. tenuis than allopatric P. nigra. The data also suggest that the inferior competitive ability of allopatric P. nigra may be a consequence of both exploitative and interference competition. Coexistence of P. nigra with P. tenuis appears to increase P. nigra's tolerance to environmental stress; the current experiments and observations during several decades of research have indicated poorer survival and growth in allopatric than sympatric populations.