The d1 mutant, which is deficient for the heterotrimeric G-protein subunit (G) gene of rice, shows dwarfism and sets small round seeds. To determine whether dwarfism in d1 is due to a reduction in cell number or to shortened cell length, the cell number of the leaf sheath, the internode, the root and the lemma was compared between Nipponbare, a wild-type rice and d1-5, a d1 allele derived from Nipponbare. Our results indicate that the cell number was reduced in all organs analyzed in d1-5. In addition, cell enlargement was found in roots and lemma of d1-5, although the organ length in d1-5 was shorter than that of wild-type rice. These results suggest that rice G participates in cell proliferation in rice. Western blot analyses using anti-G antibody and RT–PCR analyses indicate that G is mostly expressed in the developing organs. G promoter activity studies using the GUS reporter gene confirmed that the expression of G was highest in developing organs. We conclude that rice G participates in the regulation of cell number in a developmental stage-dependent manner.