The study reports the molecular cloning of the *Arabidopsis thaliana* *GAI* gene, which encodes a protein that negatively regulates gibberellin (GA) responses. The *gai* mutant allele lacks a 17-amino-acid segment from the amino-terminal region, leading to a dominant gain-of-function phenotype. The *GAI* protein contains nuclear localization signals, a LXXLL motif, and is a member of the VHHID domain family. Genetic analysis shows that *GAI* is a repressor of GA responses, and that GA can release this repression. Mutations at *SPY* and *GAR2* suppress the *gai* phenotype, indicating their involvement in a signaling pathway that regulates GA responses. The existence of this pathway suggests that GA modulates plant growth through derepression rather than simple stimulation. The *GAI*, *SPY*, and *GAR2* genes define a system for GA-mediated growth regulation that is likely common to all higher plants.The study reports the molecular cloning of the *Arabidopsis thaliana* *GAI* gene, which encodes a protein that negatively regulates gibberellin (GA) responses. The *gai* mutant allele lacks a 17-amino-acid segment from the amino-terminal region, leading to a dominant gain-of-function phenotype. The *GAI* protein contains nuclear localization signals, a LXXLL motif, and is a member of the VHHID domain family. Genetic analysis shows that *GAI* is a repressor of GA responses, and that GA can release this repression. Mutations at *SPY* and *GAR2* suppress the *gai* phenotype, indicating their involvement in a signaling pathway that regulates GA responses. The existence of this pathway suggests that GA modulates plant growth through derepression rather than simple stimulation. The *GAI*, *SPY*, and *GAR2* genes define a system for GA-mediated growth regulation that is likely common to all higher plants.