The rumen bacterium Butyrivibrio fibrisolvens was able to grow in vitro when timothy grass fructan, inulin, inulooligosaccharides, sucrose, or fructose was the sole source of carbon in the culture medium, and utilized 89.9, 47.7, 80.9, 87.9 and 94.5% of the carbohydrate, respectively. The distinctly lower utilization of inulin than other carbohydrates was accompanied by a considerably lower bacteria count in the cultures. Examination of an extract from bacterial cells showed that its fructanolytic activity was closely related to the carbon source in the growth medium. The number of fructanolytic enzymes synthesized by the bacteria depended on the carbon source in the growth medium.