Hitherto used characters for the separation of Agabus lapponicus (Thomson) from A. congener (Thunberg) are critically reviewed. From body size and shape it is impossible to separate the 2 species when sympatric. Width of metasternal wings and structure of male protarsus show great individual variation or are largely correlated to size. It is concluded that the 2 species can only be separated by the differently shaped apical part of the penis. In A. congener, the apical projection is distinctly shorter than the maximum width of the apical part. In A. lapponicus the projection is longer or subequal to the maximum width. When quantified as a width/length ratio, the frequency distribution of this character for 191 Swedish males including both species is bimodal, and this is also the case for pooled samples from the sympatric area. In this area each local population sample tends to represent only 1 of the 2 species, though coexistence was also observed. The Swedish distributions are outlined. Material from other parts of Europe shows that A. lapponicus has a boreoalpine distribution. It occurs at high altitude sites in the Alps and the Pyrenees. A lectotype is designated for Agabus congener (Thunberg).