Abstract
The semi-aquatic freshwater earthworm genus Glyphidrilus Horst, 1889 from Thailand was investigated based on extensive recent collecting. The species in this genus were characterized by their external and internal morphological characters of the location of wings, genital openings, genital organ structures and their locations, as well as the dimensions of body length and number of segments. Several type specimens were compared with both previous and newly collected materials. Ten new species are described from several river systems in Thailand; as Glyphidrilus borealis sp. n., Glyphidrilus chaophraya sp. n., Glyphidrilus chiensis sp. n., Glyphidrilus huailuangensis sp. n., Glyphidrilus kratuensis sp. n., Glyphidrilus quadratus sp. n., Glyphidrilus trangensis sp. n., Glyphidrilus wararamensis sp. n., Glyphidrilus vangthongensis sp. n. and Glyphidrilus vesper sp. n. Each species is endemic to a single river system. All 26 previously described species are re-described, and eight lectotypes have been designated. An identification key and a morphological comparison summary are provided.