Gibberellins (GAs) regulate diverse aspects of plant growth and development. Despite extensive analysis of the GA-metabolic pathway, only a few genes have been identified as regulatory components of GA metabolism. In searching for those genes, we screened and isolated a novel dominant mutant, GA-sensitive dwarf1-1D(gsd1-1D), from Arabidopsis thalinana. This mutant exhibited the characteristic phenotypes of GA-deficient mutants, including semi-dwarfism, dark-green leaves, late-flowering, and reduced fertility. Exogenously applied GA rescued the gsd1-1D mutant phenotypes, implying that this phenomenon was likely due to a reduced level of CA. Likewise, transcripts of GA-responsive genes were affected by this gsd1-1D mutation, which genetic analysis showed to be semi-dominant and monogenic. Chromosomal mapping of the GSD1 locus indicated that it resides on the middle of Chromosome 3, where no loci related to GA metabolism exist. These results suggest that the GSD1 locus encodes a novel regulatory component controlling the bioactive GA level in A. thaliana.