Highest production of xylose Isomerase by Neurospora crassa grown with different carbon sources was at 0.014 U mg(-1) with D-xylose. The enzyme exhibited maximum activity at pH 8.0 and 70 degrees C and retained 100% activity at 45 degrees C for 30 min at pH 8.0. It was activated by 8 mM Mg2+ whereas 2 mM Co2+ afforded protection against inactivation by heat. The K-m for xylose was 10 mM and 22 mM for xylose isomerase and xylose reductase respectively at 28 degrees C and pH 7.0. This Is the first report on the presence of xylose isomerase in N. crassa and the existence of two different pathways for the utilization of D-xylose.