An anaerobic syntrophic bacterial culture degrading benzoate was isolated from a river sediment. The syntrophic organism was grown in coculture in the presence of a hydrogenotrophic strain, Desulfovibrio fructosovorans or Methanospirillum hungatei. The G + C content of the syntrophic benzoate degrader determined by density gradient ultracentrifugation was similar to that of Syntrophus buswellii (54.3%). A method ensuring the G + C% determination of syntrophic bacteria is presented.