The introduction of dwarfing traits into crops was a major factor in increased grain yields during the "Green Revolution." In most cases those traits were the consequence of altered synthesis or response to the gibberellin (GA) plant hormones. Our current understanding of GA synthesis and physiology has been facilitated by the characterization of mutants. To introduce concepts about GA hormone physiology and plant transformation in an undergraduate laboratory course we have used "ga5", a semi-dwarf "Arabidopsis" mutant with reduced activity of GA 20-oxidase. In this laboratory exercise, "Arabidopsis ga5" mutant plants are transformed by the floral-dip method using "Agrobacterium tumefaciens" carrying plasmid constructs conferring kanamycin resistance and containing the "GA5" gene. Within 4 weeks, seeds of transformed plants can be easily screened by antibiotic resistance on plates. After transfer to soil the dwarf mutant plants transformed with a wild-type version of the gene show normal size. In addition to offering a visual understanding of the effect of GA on stem elongation, students learn additional techniques in this experiment, including PCR and agarose gel electrophoresis. This experiment is cost effective and can be completed within a 4-month term. (Contains 6 figures.)