Gibberellins are a classic example of the production of plant growth regulators by microorganisms. They are important biotechnological products and are increasingly used in agriculture and horticulture. The economic importance of these plant hormones has led to an extensive study of the regulation of gibberellin biosynthesis. There have been reports of light, growth rate, inoculum size and carbon and ammonium sources acting as regulators of gibberellic acid biosynthesis. Besides light stimulation, nitrogen repression is a well-known regulatory principle of secondary metabolite formation. In Gibberella fujikuroi ammonium interferes with the production of gibberellic acid whereas phosphate does not influence the biosynthesis. It was found that the negative effect of ammonium ions is due to both the inhibition of activity and the repression of de novo synthesis of specific gibberellin-producing enzymes. Besides nitrogen control, the biosynthesis of gibberellins is suppressed by glucose. This glucose effect can be overcome by the addition of mevalonic acid. Therefore, the key enzyme of the isoprenoid pathway, the HMG-CoA reductase, seems to be the target of C-catabolite repression. A detailed knowledge of the regulation of gibberellin biosynthesis is important for fermentation processes. The biological function of gibberellin formation for the producing fungus is discussed.