To investigate the signal transduction pathway of cytokinin, we incubated seven-day-old maize leaves in distilled water with or without 0.1 mM benzylaminopurine (BAP). Several gene fragments were either induced or repressed by this exogenous treatment, and were then isolated by differential display-polymerase chain reaction (DD-PCR). One fragment (IBC4) had significant sequence homology with the N-terminal portion of the chloroplast ATP synthase IV subunit gene (atpI) of higher plants. After the stimulated expression of IBC4 by BAP was confirmed using orthern blot hybridization, we isolated the full-length gene, including IBC4, from the chloroplast genome. We verified it as an atpI gene, using BLAST search analysis. This atpI gene, designated as Zm-atpI, (Zea mays-atpI), is 744 nucleotides long and encodes 247 amino acids. Its deduced amino acid sequence does not vary among maize cultivars, and it shares >90% identity with the atpI genes from Oryza sativa, Triticum aestivum, Nicotiana tabacum, Spinacia oleracea, and Pisum sativum. We also confirmed, by northern blot analysis, that expression of Zm-atpI transcripts was significantly stimulated by the exogenous application of BAP.