Bacillus subtilis coproduces several surfactin variants that are powerful biosurfactants and have potential applications in biology and industry. A single amino acid substitution in the heptapeptide moiety of surfactins strongly modifies their properties. To better establish structure-activity relationships and to search new variants with enhanced properties, Bacillus subtilis was grown into two modified culture media. Two new variants were isolated by chromatographic methods and studied by NMR spectroscopy. As planned, modifications consisted in the substitution of the l-valine residue at the fourth position by a more hydrophobic residue, i.e., leucine or isoleucine. These [Leu4]- and [Ile4]surfactins have a higher affinity for hydrophobic solvents and a twice improved surfactant power. Structure-property correlations were confirmed by analysis of the hydrophobic residue distribution in the three-dimensional model of the structure of surfactin in solution.