Serratia marcescens SS-1 and its SpnR-defective isogenic mutant, SMDeltaR, produced an extracellular surfactant able to decrease surface tension of water from 72 to 37 dyne cm(-1) (SMDeltaR strain) and to 45 dyne cm(-1) (SS-1 strain). The biosurfactant also emulsified kerosene and diesel with a maximum emulsion index of 77% ( diesel and kerosene) for the SMDeltaR strain, and 72% ( kerosene) and 40% ( diesel) for the SS-1 strain. Deletion of spnR gene appeared to enhance biosurfactant production. Model simulations suggest that biosurfactant production by the two strains was growth-associated. The SMDeltaR strain had a yield coefficient of 22 - 32% g dry cell(-1), which is 32 - 50% higher than that of the SS-1 strain.