A chitinase cDNA clone from rapeseed (Brassica napus L. ssp. oleifera) was isolated. The cDNA clone, ChB4, represents a previously purified and characterized basic chitinase isozyme. The longest open reading frame in ChB4 encodes a polypeptide of 268 amino acids. This polypeptide consists of a 24 amino acid N-terminal signal peptide, a cysteine-rich domain and a catalytic domain. The primary structure of the mature ChB4 shows a low degree of identity with class I and II chitinases, 43-48% and 35%, respectively. In contrast, ChB4 shows 62% identity to a basic sugar-beet chitinase and 63% identity to an acidic chitinase from bean. The expression of chitinase messenger RNA (mRNA) in response to infection with Phoma lingam (Tode ex. Fr.) Desm. was examined by northern hybridization and scintilation counting. A differential induction was seen between resistant and susceptible cultivars where 3-fold higher chitinase transcript levels were estimated one day after inoculation in resistant as compared to susceptible cultivar. This difference diminished eight days after inoculation. Southern hybridization analysis indicates that the chitinase is encoded by a small family of genes.