Methods for isolation of fecal 7 alpha-dehydroxylating bacteria are presented. A total of 219 strains were isolated from feces of healthy humans, and their ability to 7-dehydroxylate cholic, chenodeoxycholic, and ursodeoxycholic acids were examined. Of all the isolates, 14 strains were found to be capable of eliminating the hydroxy group at C-7 alpha and/or C-7 beta. All the isolates were strictly anaerobic, Gram-positive rods. Thirteen isolates were non-sporeforming bacteria showing certain saccharolytic properties with the production of acid and gas from dextrose, and were catalase-positive but indole-, lecithinase-, urease- and oxidase-negative. Based on the data available at present, it was concluded that they could be regarded as members of the genus Eubacterium. One strain, however was identified as Clostridium sordellii. The isolated strains capable of 7 alpha-dehydroxylating cholic acid and chenodeoxycholic acid were also able to oxidize the hydroxy group at C-7 alpha. Nine strains (10, 12, 36S, M-2, M-17, M-18, Y-98, Y-1112, and Y-1113) of the 7 alpha-dehydroxylating bacteria were confirmed to have 7 beta-dehydroxylation ability, but five strains (O-51, O-52, O-71, O-72, and Y-67) could not transform ursodeoxycholic acid to lithocholic acid.