Although both the a-amylase super-family, i.e. the glycoside hydrolase (GH) clan GH-H (the GH families 13, 70 and 77), and family GH31 share some characteristics, their different catalytic machinery prevents classification of GH31 in clan GH-H. A significant but remote evolutionary relatedness is, however, proposed for clan GH-H with GH31. A sequence alignment, based on the idea that residues equivalent in the primordial catalytic GH-H/GH31 (b/a) 8 -barrel may not be found in the present-day GH-H and GH31 structures at strictly equivalent positions, shows remote sequence homologies covering b3, b4, b7 and b8 of the GH-H and GH31 (b/a) 8 -barrels. Structure comparison of GH13 a-amylase and GH31 a-xylosidase guided alignment of GH-H and GH31 members for construction of evolutionary trees. The closest sequence relationship displayed by GH31 is to GH77 of clan GH-H.