The invention relates to a multifunctional germacrene-D synthase and a polynucleotide encoding it. Included also are genetic constructs, transgenic organisms, plant selection methods and biofermentation methods. The enzyme may be used to prepare germacrene-D and/or other sesquiterpenes such as germacrene-D, delta-cadinene, gamma-cadinene, gamma-muurolene, gamma-elemene, delta-elemene, elemol or germacrene-B.