The invention refers to a new microbial process for preparing .beta.,.delta.-dihydroxy-7-[1,2,6,7,8,8a-hexahydro-2.6-dimethyl-8-(2-methyl-butyryloxy)-naphthalen-1-yl]-heptanoic acid .delta.-lactone of formula (I) and .beta.,.delta.-dihydroxy-7-[1,2,6,7,8,8a-hexahydro-2.6-dimethyl-8-(2-methylbutyryloxy)-naphtalen-1-yl]-heptanoic acid of formula (II), ##STR1## by the aerobic fermentation of the submerged culture of an imperfect fungus strain biosynthesizing the above compounds, in a nutrient medium containing utilizable carbon and nitrogen sources as well as mineral salts, and by isolating the product of formula (I), which comprises cultivating a strain of the new Aspergillus obscurus fungus species producing the above compound(s) of formulas (I) and/or (II), in a temperature range of 25.degree. to 30.degree. C. and, if desired, separating the product(s) formed from the fermentation broth, then isolating it in the lactone form of formula (I) and, if desired, purifying the same.