The phytohormone abscisic acid (ABA) is involved in the adaptive stress response and regulates expression of many stress-responsive genes, including some transcriptional factors. A bZIP transcription factor, OsABI5, was isolated from the panicle of Oryza sativa L. Expression of the OsABI5 gene was induced by abscisic acid (ABA) and high salinity, and down-regulated by drought and cold (4 degrees C) in seedlings. The OsABI5 protein was localized in the nucleus and has trans-activation activity. The N-terminal of the protein is necessary for its activity. OsABI5 could bind to a G-box element and trans-activate reporter gene expression. Complementation analysis revealed that the expression of OsABI5 driven by the 35S promoter could rescue ABA-insensitivity of abi5-1 during seed germination and result in hypersensitivity to ABA. Over-expression of OsABI5 in rice conferred high sensitivity to salt stress. Repression of OsABI5 promoted stress tolerance and resulted in low fertility of rice. These results suggested that OsABI5 could regulate the adaptive stress response and plant fertility of rice as a transcription factor.