The cell cycle of eukaryotes is tightly regulated through the activity of cyclin-dependent kinases. The Arabidopsis thaliana CDKA;1 (CDC2aAt) gene is thought to encode such a protein kinase, since it is actively transcribed in proliferating tissues and can complement defects in the Schizosaccharomyces pombe cdc2 gene. We analyzed the functional structure of the CDKA;1 promoter, using fusion genes between various upstream regions of CDKA;1 and the Escherichia coli beta-glucuronidase (GUS) gene. A 595 bp DNA fragment upstream from the transcription start site conferred GUS activity on developing trichomes, but not on proliferating tissues. On the other hand, another upstream fragment extending to the 5' non-coding transcribed region gave GUS activity to both proliferating tissues and developing trichomes. Against the gl2 mutant background, GUS activity directed by the 595 bp fragment was detected in single-stalk cells, but not in giant cells without obvious polar extension growth. These results revealed that the 595 bp fragment lacks cis element(s) essential for proliferating-cell-specific promoter activity, but can direct transcription in a specific period during trichome development, which does not include cell division. This suggests that CDKA;1 functions during cell morphogenesis as well as cell proliferation.