SUMMARY From a large number of cultures of cellulolytic bacteria isolated from rumen contents of sheep conditioned to low-protein teff hay through the use of both a selective medium containing finely ground cellulose as energy source and a less selective medium containing cellobiose, xylan and starch, thirty isolates, representing three morphological types, were selected for detailed charac- terization. Nineteen isolates of Gram-negative curved rods belonged to the genus Butyrivibrio. Of these one was identical with and ten closely related to B. Jibrisolvens. The remaining eight Butyrivi brio cultures differed in several respects from the only other defined species within this genus, B. alactacidi- gens. Four coccal isolates were identified as Rminococcus albus and five as R.javefaciens. Two strains of sporeformers belonged to the genus Clostridium but could not be identified with any of the cellulolytic species of this genus listed by Bergey's Manual.