Complementary DNA encoding a putative cyclobutane pyrimidine dimer (CPD)-specific DNA photolyase (CPD photolyase) was isolated from cucumber leaves. The deduced amino acid sequence of the cDNA exhibited high similarity to that of the Arabidopsis CPD photolyase. Transformation with the cDNA restored the impaired photorepair activity of an Escherichia coli mutant, indicating that this cDNA encodes a functional cucumber CPD photolyase (CsPHR). The level of CsPHR transcripts estimated by quantitative RT-PCR as well as the CPD photorepair activity in the cucumber first leaves showed diurnal changes, peaking at 09 : 00 and 12 : 00, respectively. Supplemental UV-B irradiation in the middle of the light period had little effect on the growth of the first leaves, while the supplemental irradiation in the early morning or late afternoon strongly retarded the leaf growth. These results suggest that the diurnal change in CPD photorepair activity, which is presumably regulated by the transcript level of CsPHR, may play an important role in minimizing the growth inhibition due to UV-B irradiation.