Several cultures of bacteria, isolated from the rumen, that were able to utilize plant cell wall structural polysaccharides were grown on a range of carbohydrate substrates and the activities of the principal polysaccharide-degrading enzymes determined. The esterase activity was also monitored. The extent of hemicellulose degradation and utilization by the isolates was comparable with that of the hemicellulolytic type strains. Enzyme activities in all of the cultures examined were affected by the carbon source in the growth medium. Many responses were strain specific, although growth on glucose (or cellobiose and maltose to a lesser extent) resulted in reduced activities in most of the organisms examined, whilst polysaccharidic substrates resulted in higher levels of the appropriate polysaccharidase. However, enzyme activity was detectable in some isolates after culture on mono- or disaccharides in the absence of the principal or related polysaccharide substrate.