The CURLY LEAF (CLF) gene in Arabidopsis thaliana (L). Heynh. is required for stable repression of a floral homeotic gene, AGAMOUS in leaves and stems To clarify the function of CLF in organ development, we characterized clf mutants using an anatomical and genetic approach. The clf mutants had normal roots, hypocotyls, and cotyledons, but the foliage leaves and the stems had reduced dimensions. A decrease both in the extent of cell elongation and in the number of cells was evident in the clf mutant leaves, suggesting that the CLF gene might be involved in the division and elongation of cells during leaf morphogenesis. An analysis of the development of clf mutant leaves revealed that the period during which tell division or cell elongation occurred was of normal duration, while the rates of both cell production and cell elongation were lower than in the wild type. Two phases in the elongation of cells were also recognized from this analysis. From analysis of an angustifolia clf double mutant, we found that the two phases of elongation of leaf cells were regulated independently by each gene. Thus, the CLF gene appears to affect cell division at an earlier stage and cell elongation throughout the development of leaf primordia.