Aims: The objective was to determine the effect of the isoflavone biochanin A (BCA) on rumen cellulolytic bacteria and consequent fermentative activity. Methods and Results: When bovine microbial rumen cell suspensions (n = 3) were incubated (24 h, 39 degrees C) with ground hay, cellulolytic bacteria proliferated, short-chain fatty acids were produced and pH declined. BCA (30 mu g ml(-1)) had no effect on the number of cellulolytic bacteria or pH, but increased acetate, propionate and total SCFA production. Addition of BCA improved total digestibility when cell suspensions (n = 3) were incubated (48 h, 39 degrees C) with ground hay, Avicel, or filter paper. Fibrobacter succinogenes S85, Ruminococcus flavefaciens 8 and Ruminococcus albus 8 were directly inhibited by BCA. Synergistic antimicrobial activity was observed with BCA and heat killed cultures of cellulolytic bacteria, but the effects were species dependent. Conclusions: These results indicate that BCA improves fibre degradation by influencing cellulolytic bacteria competition and guild composition. Significance and Impact of the Study: BCA could serve as a feed additive to improve cellulosis when cattle are consuming high-fibre diets. Future research is needed to evaluate the effect of BCA on fibre degradation and utilization in vivo.