Abstract: A microbiological process for the production of .gamma.-decalactone. According to this process, a culture of the fungus of a particular species of the genus Mucor is incubated with the particular substrate, which is an organic carboxylic acid ester, to produce fermentatively the .gamma.-decalactone. Exact conditions have now been discovered under which .gamma.-decalactone can be prepared in high yields. These conditions are based upon the fermentative incubation of a fungus of the genus Mucor in conjunction with the appropriate substrate in order to obtain the high yield of the desired lactone.