One hundred and nine srains of obligate anaerobic bacteria isolated from the intestines of 6freshwater fish were examined for morphological, physiological and biochemical characteristics. They were divided into two distinct groups, A and B, which consisted of two single species, judging from the consistency of most characteristics within each group. They were non-motile, non-sporing, gram-negative, obligate anaerobic rods which produced acetic acid but not butyric or lactic acid as major products in PYFG broth. Based on these rsults, both groups should belong to the genus Bacteroides. Most of the obligate anaerobic strains demonstrated no activity or weak activity in macromolecule hydrolysis and carbohydrate fermentation compared with facultative anaerobes isolated from fish intestines.