SUMMARY: A mutant of Aspergillus niger unable to grow on d-xylose and l-arabinose has been isolated. Genetic analysis revealed that the mutation is located on linkage group IV. Enzymic analysis revealed a deficiency in d-xylulose kinase activity. After transfer of growing mycelium to a medium containing either d-xylose or l-arabinose, the mutant accumulates large amounts of arabitol and xylitol, as shown by 13C NMR spectroscopy. These data and an analysis of enzyme activities induced by d-xylose and l-arabinose in the wild-type strain led to the following catabolic pathway for d-xylose: d-xylose - xylitol - d-xylulose - d-xylulose 5-phosphate; and for l-arabinose: l-arabinose - l-arabitol - l-xylulose - xylitol - d-xylulose - d-xylulose 5-phosphate. The reduction steps of the sugars to the corresponding polyols are all NADPH dependent. The oxidation steps of the polyols to the sugars are all NAD+ dependent. Fractionation of cell-free extracts gave information about the specificity of the enzymes and showed that all the reactions are catalysed by different enzymes.