The ruminal bacterium, Prevotella bryantii B(1)4, grew more rapidly with glucose as an energy source than rnannose (0.73 versus 0.47 h(-1)) and had 8-fold less beta-glucanase activity (50 versus 400 nmol reducing sugar mg protein(-1) min(-1)). Cultures that were provided with glucose and mannose had little beta-glucanase activity even though both sugars were utilized simultaneously. The observation that glucose and mannose were utilized simultaneously indicated that P-glucanase expression was not merely a simple induction or inducer exclusion. When glucose was added to cultures growing on rnannose, hexose flux through the glucomannokinase increased 1.5-fold, and this increase was associated with an almost immediate decrease in beta-glucanase mRNA. After only three generation (doubling) times, the amount of beta-glucanase mRNA was comparable to that observed in cells rowing only with glucose. These results indicate that beta-glucanase activity is transcriptionally regulated. However, further work will be needed to define more precisely the nature of this regulation and to identify the intermediate in this response.