Brassinosteroids (BRs) are plant growth-promoting natural products required for plant growth and development. Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of the lamina joint of rice. However, little is known about the function of endogenous BR in rice or other grass species. We report here the phenotypical and molecular characterization of a rice dwarf mutant, d61, that is less sensitive to BR compared to the wild type. We cloned a rice gene, OsBRI1, with extensive sequence similarity to that of the Arabidopsis BRI gene, which encodes a putative BR receptor kinase. Linkage analysis showed that the OsBRI1 gene is closely linked to the d61 locus. Single nucleotide substitutions found at different sites of the d61 alleles would give rise to amino acid changes in the corresponding polypeptides. Furthermore, introduction of the entire OsBRI1 coding region, including the 5' and 3' flanking sequences, into d61 plants complemented the mutation to display the wild-type phenotype. Transgenic plants carrying the antisense strand of the OsBRI1 transcript showed similar or even more severe phenotypes than those of the d61 mutants. Our results show that OsBRI1 functions in various growth and developmental processes in rice, including (1) internode elongation, by inducing the formation of the intercalary meristem and the longitudinal elongation of internode cells; (2) bending of the lamina joint; and (3) skotomorphogenesis.