A novel lipoxygenase cDNA (3,007 base pairs) was isolated from rice leaves (Oryza sativa cv. Aichiasahi) which had been infected with an incompatible race of the rice blast fungus, Magnaporthe grisea. A single copy of the gene is present in the rice genome and encodes a protein of 923 residues with a molecular weight of 102,714. This gene product shares the least amino acid sequence homology among plant lipoxygenases identified to date. A novel feature of this gene product is a putative transit peptide sequence at the amino terminus, suggesting the enzyme is localized in chloroplasts. An active lipoxygenase was expressed from the cDNA in Escherichia coli and characterized. The lipoxygenase introduces molecular oxygen exclusively into the C-13 position of linoleic and linolenic acids. The gene is expressed at high levels 15 h after inoculation with an incompatible race of M. grisea, at a low level after inoculation with a compatible race of the pathogen, and is not expressed in mock-infected leaves. Gene expression begins at the same time that the pathogen begins to penetrate into leaf tissue. This novel lipoxygenase gene expression is a part of the early response of the host to pathogenic attack.