Tomato (Lycopersicon esculentum Mill.) plants homozygous for the mutant pro gene, exhibiting the distinctive procera phenotype, appeared virtually identical to gibberellic acid (GA3)-treated isogenic normal plants. The pro gene and GA3 caused analogous increases in internode length, and in the length and number of cells in the outer cell layers of each internode. Internode number was also increased by pro and GA3 over the period of the experiment. Despite their greater length, the internodes of GA3-treated and pro plants reached their final size within a time period similar to that of internodes of untreated normal plants. The pro mutant itself was responsive to GA3, especially in the seedling stage, but the proportional increase in height seen in the later stages of growth was less than that of normal plants.