The amount of active gibberellin (GA) in plant tissues is determined in part by its rate of catabolism through oxidation at C-2. In pea (Pisum sativum L.) seeds, GA 2-oxidation is controlled by the SLN (SLENDER) gene, a mutation of which produces seedlings characterized by a slender or hyper-elongated phenotype. We cloned a GA 2-oxidase cDNA from immature pea seeds by screening an expression library for enzyme activity. The clone contained a full-length open reading frame encoding a protein of 327 amino acids. Lysate of bacterial cultures expressing the protein converted the C(19)-GAs, GA(1), GA(4), GA(9), and GA(20) to the corresponding 2beta-hydroxy products. GA(9) and GA(20) were also converted to GA(51) and GA(29) catabolites, respectively. The gene appeared to be one member of a small family of GA 2-oxidases in pea. Transcript was found predominantly in roots, flowers, young fruits, and testae of seeds. The corresponding transcript from sln pea contained a point mutation and did not produce active enzyme when expressed heterologously. RFLP analysis of a seedling population segregating for SLN and sln alleles showed the homozygous mutant allele co-segregating with the characteristic slender phenotype. We conclude that SLN encodes GA 2-oxidase.