[en] A homologous series of surfactins containing β-hydroxy fatty acids having 13, 14, or 15 carbon atoms were isolated from the supernatant of Bacillus subtilis strain S499 cultures. Their surface-active properties at the air-water interface were then evaluated. Dynamic surface tension data were analyzed by the relaxation function γt=γm+(γo−γm)/[1+(t/t*)n]. Based on various parameters t*, n, vmax, γm calculated from this equation, the dynamic surface properties of surfactin were found to depend on both bulk concentration and hydrophobic character of the alkyl chain. At low concentrations of surfactin, the dynamic surface tension (γd) decreased with increasing carbon atom number of the surfactin alkyl chain (n=13 to 15). However, at high concentrations, the maximum decrease of 41-4 was achieved by surfactin-C14. In contrast, more strongly hydrophobic alkyl chains in surfactins always enhanced their ability in reducing the equilibrium surface tensions and their aptitude in forming micelles.