Based on amino-acid sequence data from Aspergillus niger alpha-L-arabinofuranosidase B (ABF B), and cyanogen bromide fragments derived thereof, deoxyoligonucleotide mixtures were designed to be employed as primers in a polymerase chain reaction (PCR) on A. niger genomic DNA. This resulted in amplification of three related PCR products. The abfB gene encoding ABF B was isolated from a genomic library using such an amplification product as a probe. A 5.1-kb BamHI fragment was subcloned to result in plasmid pIM991. Upon introduction by co-transformation into both A. niger and A. nidulans uridine auxotrophic strains, pIM991 was shown to contain the functional gene since prototrophic transformants overproduced ABF B upon growth on the inducing carbon source sugar beet pulp. A plate assay was developed enabling quick selection of ABF B-overproducing transformants. The sequence of a 4122-bp long BamHI/SstI fragment was determined. The abfB gene does not contain introns and codes for a protein of 499 amino acids. The mature ABF B, 481 amino acids in length, has a deduced molecular weight of 50.7 kDa. A. niger abfB is the first eukaryotic gene encoding an ABF to be characterized.