Mutants of colicin A have been prepared in which the three tryptophan residues (Trp86, Trp130 and Trp140), localized in the C-terminal domain of the soluble wild-type protein, have been substituted by phenylalanine. The Trp140Phe single mutation had the effect of decreasing the percentage of protein that is expressed as insoluble aggregates. The created hydrophobic cavity decreased the stability of the protein during its folding, resulting in partial aggregation in the cytoplasm of the Escherichia coli-producing cells. Aggregation was increased when Trp140 was substituted by Lys, Leu or Cys, or if the Trp140 mutation was combined with the Trp86Phe and/or Trp130Phe mutations. A single mutation, Lys113Phe, however, was able to restore the solubility of the aggregated mutants in vivo. Detailed analysis of the 3-D structure of the C-terminal domain of colicin A suggests that filling of the hydrophobic cavity is responsible for this effect.