Stereoselective preparation procedure of gamma-lactones, characterized in that microbial biosynthesis of gamma-lactones of general formula (I) is performed in which the lactonic cycle carries an unsaturation between carbon # 2 and carbon # 3 o is saturated, and in which R1 is a C1-16 alkenyl, C1-16 alkynyl or C1-16 alkyl, optionally substituted, said biosynthesis being carried out from a fatty acid, with the aid of a microbial culture of a strain of genus Aspergillus sp. or Mortierella sp., allowing a stereoselective hydroxylation in C4 of said substrate, said procedure comprising the following steps: a) selecting a strain of the genus Aspergillus sp. or Mortierella sp., b) culturing said strain in a culture medium free of peptones, the cell culture obtained having a "compote type" mycelium composed of compartmentalized filaments without conidiospores and having swollen structures, full of inclusions, said culture being possibly preceded by a step of previously cultivating the strain, c) adding a fatty acid capable of transforming into gamma-lactone, d) bioconverting the fatty acid into gamma-lactone of formula (I), e) recovering the produced gamma-lactone.Procedimiento de preparación estereoselectiva de gamma-lactonas, caracterizado porque se realiza una biosíntesis por vía microbiana de gamma-lactonas de fórmula general (I) en las que el ciclo lactónico lleva una insaturación entre el carbono n.º 2 y el carbono n.º 3 o está saturado, y en las que R1 es un alquenilo C1-16, alquinilo C1-16 o alquilo C1-16, eventualmente sustituido, realizándose dicha biosíntesis a partir de un ácido graso, con ayuda de un cultivo microbiano de una cepa del género Aspergillus sp. o Mortierella sp., permitiendo una hidroxilación estereoselectiva en C4 de dicho sustrato, comprendiendo dicho procedimiento las siguientes etapas: a) seleccionar una cepa del género Aspergillus sp. o Mortierella sp., b) cultivar dicha cepa en un medio de cultivo exento de peptonas, presentando el cultivo ce
