(-)-Geosmin (1 blue) is an important odor component produced by many bacteria including actinomyceles myxobacteia and cyanobacteria but has also been reported for eukaryotic organisms such as fungi, liverworts, insects, and plants. Recent research has shown that the biosynthesis of 1 starts with the cyclization of famesyl pyrophosphate to (1(10)E,5E)-germacradien 11-ol (2, yellow), the first key intermediate en route. In a retro-Prins-reaction acetone is lost, and the second key intermediate, (8S,9S,10S)-8,10dimethyl-1-octalin (3, red), is formed. After reprotonation, a 1,2-H-shift and attack of water. geosmin is finally released. Octalin3 occurs in the bouquets of volatiles released by myxobacleria as well as in enzyme extracts of incubation experiments of the purified geosmin synthase from Streptomyces coelicolor. Here the identification of 3, a new natural compound, the elucidation of its stereochemistry. and its synthesis are reported.