The filamentous fungus, Penicillium chrysogenum, is able to grow on xylan as a sole carbon source. Under these conditions, high levels of a xylanase (XYLP) are secreted into the medium. After purification and characterization of this enzyme, we have isolated both the encoding cDNA and the genomic sequence by using oligodeoxyribonucleotides derived from partial amino acid (aa) sequences of the purified enzyme. The gene is approximately 1.6 kb in length, and comparison of the nucleotide (nt) sequence of the genomic and the cDNA clone revealed the presence of ten exons and nine introns. All intron/exon splice junctions exactly follow the GT/AG rule, except for the seventh intron which shows atypical AT/AC splice sites. The immediate 5'-flanking region of the first exon contains one putative CCAAT consensus sequence and a perfect TATA box. Primer extension analysis revealed two transcription start points located 38 and 34 nt upstream from the ATG start codon. A sequence of 23 aa representing a typical signal peptide is present at the N terminus of the deduced aa sequence. Northern blot analysis of total cellular RNA indicated that xylP encodes a 1.3-kb transcript which is induced by xylan. The aa sequence of XYLP shows considerable homology to high-M(r) acidic xylanases (Xln) and cellulases from different bacteria, yeasts and fungi.