Overexpression of bovine somatotropin (bST) results in the formation of inclusion bodies, leading to abnormal elongation of the Escherichia coli cell. As the cell elongates, the fraction of the inclusion body in the cell decreases and, consequently, bST productivity is lowered. Delayed expression of bST can be expected to prevent cell elongation; however, the trp promoter is leaky and incompletely repressed by tryptophan. The addition of 15 g/l yeast extract, which contained various aromatic amino acids, into the starting medium delayed the derepression of the trp promoter significantly, whereas the addition of 100 mg/l tryptophan did not. With the delayed expression, no cell elongation was observed until harvest and the final bST productivity increased more than three-fold. Intracellular tryptophanase, which degrades tryptophan, is another factor influencing regulation of the trp promoter. Expression of the tryptophanase operon is known to be subject to catabolite repression. The addition of a monosaccharide such as glucose, fructose, or galactose into the seed culture medium delayed the derepression of the trp promoter further in the main fed-batch fermentation.