Sophorolipids from the non-pathogenic yeast Candida bombicola are applied commercially as biodegradable, eco-friendly surface active agents. These sophorolipids are produced by cultivation in presence of a hydrophobic carbon source and are always constituted of a mixture of structurally related molecules. For some applications however, certain structural variants perform better than others. Acetylation of the sophorolipid molecule is such a parameter that gains interest because of its influence on water solubility, foaming properties, and biological activity. Fully unacetylated sophorolipids therefore are interesting metabolites but cannot be produced in a pure way by conventional cultivation. Here we report the identification of the acetyltransferase gene AT, responsible for acetylation of de novo synthesized sophorolipids in Candida bombicola. By the creation of a Delta at deletion mutant, we could create a yeast strain producing purely unacetylated sophorolipids with a yield of 5 +/- 0.7 g/L using rapeseed oil as hydrophobic carbon source. In contrast to the chemical production of unacetylated sophorolipids used nowadays, the microbial production leads to mainly lactonic sophorolipids, in addition to minor amounts of acidic sophorolipids. Biotechnol. Bioeng. 2011; 108: 2923-2931. (C) 2011 Wiley Periodicals, Inc.