Germacrene A (3) has been proposed to be an intermediate in the cyclization of farnesyl diphosphate (1) to the sesquiterpene hydrocarbon (+)-aristolochene (2). Under normal circumstances, however, neither 3 nor any other intermediate is released from the cyclase active site. When the anomalous substrate (7R)-6,7-dihydrofarnesyl diphosphate (5) is incubated with aristolochene synthase from Aspergillus terreus, the resultant dihydrogermacrene (6) cannot be further cyclized and is released from the active site. The structure of the abortive cyclization product, mit 206, was confirmed as dihydrogermacrene (6) by direct CC-MS comparison with an authentic synthetic sample prepared from (-)-(3S)-beta-citronellol.