Penicillium purpurogenum produces several endoxylanases, two of which (XynA and XynB) have been purified and characterized. XynB has been sequenced, and it belongs to glycosyl hydrolase family 11. In this publication we report the structure of the xynA gene. The amino terminal sequence of the protein was determined and this allowed the design of oligonucleotides for use in polymerase chain reactions. Different polymerase chain reaction strategies were used to amplify and sequence the entire cDNA and the gene. The gene has an open reading frame of 1450 base pairs, including 8 introns with an average length of 56 base pairs each. Only one copy of this gene is present in the P. purpurogenum genome as shown by Southern blot. The gene encodes a protein of 329 residues (including the signal peptide), and the calculated molecular mass of the mature protein is 31,668 Da. Immunodetection assays of the expressed gene positively identified it as xynA, and sequence alignments indicate a high degree of similarity with family 10 endoxylanases. It is concluded that P. purpurogenum produces endoxylanases of family 10 and 11. The complementary action of endoxylanases of both families may be important for an efficient degradation of xylan by the fungus.