A cDNA encoding a high mobility group B (HMGB) protein was isolated from Cucumis sativus and characterized with respect to its sequence, expression and responses to various abiotic stress treatments. The predicted polypeptide of 146 amino acid residues contains characteristic features of HMGB family proteins including the N-terminal basic region, one HMG-box and a stretch of acidic amino acid residues at the C-terminus. In vitro nucleic acid-binding assay revealed that the HMGB protein bound to both single-stranded DNA and double-stranded DNA. DNA gel blot analysis indicated that the HMGB gene is a single copy gene in cucumber genome. RNA gel blot analysis showed that the cucumber HMGB was more abundantly expressed in the roots than in shoots and leaves. Various abiotic stresses, including cold, drought and high salinity, down regulated markedly the expression of the HMGB in cucumber. The present report identifies a novel gene encoding HMGB protein in cucumber that shows a significant response to abiotic stress treatments.