The Namaqua rock mouse Aethomys namaquensis, a rodent pollinator of certain geoflorous Protea species, consumes nectar containing xylose. Xylose is not known to be efficiently utilized by mammals. However, it is fermented by certain bacteria, yeasts, and fungi, particularly gastrointestinal bacteria. The end products of microbial fermentation are utilized by the host in oxidative metabolism. Here we investigate the degree to which intestinal bacteria of A. namaquensis contribute to xylose metabolism. Mice were caught during Protea humiflora flowering and nonflowering seasons and given an oral dose of 14C-labeled xylose. Exhaled CO2 and excreted urine and feces were continuously collected for 30 h thereafter, and label recovery was determined. Each mouse was then treated with antibiotics to reduce gut microflora, and the experiment was repeated. With their natural gut flora population intact, mice caught during the flowering season exhaled significantly more 14CO2 than did mice caught during the nonflowering season. Also, during both seasons, mice exhaled significantly more 14CO2 before antibiotic treatment than after. Antibiotic treatment caused a significant increase in the proportion of 14C-labeled xylose that was excreted in the urine. The mouse diet likely influences the composition of the gastrointestinal community. Aethomys namaquensis relies on its gut microflora to ferment xylose, thereby converting it into end products that are used by the mice for metabolism.