Biotransformations of sesquiterpenoids by Rhizopus species have been used to provide new derivatives with potential biological activities. In these fungal transformation processes, the main action of the Rhizopus genus has been the introduction of hydroxyl groups into remote positions of the molecules, which is difficult to achieve by chemical means. Other reactions carried out with stereo-selectivity by these microorganisms have been: epoxidations of double bonds, hydrogenations of the exocyclic C-C double bond of the lactone rings, and reductions of carbonyl groups giving Salcohols. Several reactions have also been performed by these fungi with regio-selectivity, such as oxidations of hydroxyl groups and deacetylations. In some biotransformation processes, a carbon skeleton rearrangement of the substrate has taken place.