We have identified a genomic DNA fragment from the lignin-degrading fungus Phanerochaete chrysosporium (P.c.) that hybridizes to a DNA probe encoding part of the exo-cellobiohydrolase I (CBHI) gene of Trichoderma reesei (T.r.). This fragment has been subcloned and its nucleotide sequence determined. We demonstrate that it could encode a 516 residue protein that shows strong homology with the known protein sequence of CBHI from T.r. Comparison of the two nucleotide sequences identifies two regions within the P.c. sequence that are not represented in T.r. Further inspection of these regions reveals sequences closely related to the conserved elements of filamentous fungal introns. We conclude that the P.c. genomic sequence contains the same number of introns (2) as found in T.r. but that these are located at different relative positions within the two genes. The transcript from the P.c. sequence is induced in the presence of cellulose but not by glucose and we therefore conclude that this sequence represents the first cellulase gene to have been described from this organism.