The production of organic acids by two anaerobic ruminal bacteria, Fibrobacter succinogenes S85 and Ruminococcus flavefaciens FD-1, was compared with glucose, cellobiose, microcrystalline cellulose, Walseth cellulose (acid swollen cellulose), pulped paper, and steam-exploded yellow poplar as substrates. The major end product produced by F. succinogenes from each of these substrates was succinate (69.5-83%), the principal secondary product was acetate (16-30.5%). Maximum succinate productivity ranged from 14.1 mg/L.h for steam-exploded yellow Poplar to 59.7 mg/L.h for pulped paper. For R. flavefaciens, the major end product from cellobiose, microcrystalline cellulose, and acid-swollen Walseth cellulose was acetate (39-46%), pulped paper and steam-exploded yellow poplar yielded succinate (42-54%) as the major product. Maximum succinate productivity by R. flavefaciens ranged from 9.21 mg/L.h for cellobiose to 43.1 mg/L.h for pulped paper. In general, much less succinate was produced at a lower maximum productivity by R. flavefaciens than by F. succinogenes under similar fermentation conditions. The maximum succinate productivities by these two organisms are comparable to the previously reported value of 59 mg/L.h for Anderobiospirillum succiniciproducens grown on glucose and corn steep liquor.