Specific oligonucleotide probes targeted to sites on the 16S rRNA of Ruminococcus albus 8, Ruminococcus flavefaciens FD-1, and Fibrobacter succinogenes S85 and a domain Bacteria probe were used to study bacterial interactions during the fermentation of cellulose and alkaline hydrogen peroxide-treated wheat straw in monocultures, dicultures, and tricultures. Results showed that R. albus 8 inhibited the growth of R. flavefaciens FD-1 when grown as a diculture with cellulose or alkaline hydrogen peroxide-treated wheat straw as the carbon source. In dicultures containing R. albus 8 and F. succinogenes S85 grown on cellulose or alkaline hydrogen peroxide-treated wheat straw, competition was not detected. R. flavefaciens FD-1 outcompeted F. succinogenes S85 when cellulose was used as the carbon source. In tricultures with cellulose as the carbon source, R. flavefaciens FD-1 was inhibited, R. albus 8 appeared to dominate during the early phase of degradation (12 to 48 h), while F. succinogenes S85 became predominant during the later phase of degradation (60 to 70 h). When alkaline hydrogen peroxide-treated wheat straw was used as a growth substrate, F. succinogenes S85 showed better growth than either R. albus 8 or R. flavefaciens FD-1. However, R. flavefaciens FD-1 was present in small numbers throughout the incubation period, unlike the growth patterns when cellulose was the carbon source.