The Arabidopsis genome project has recently reported sequences with similarity to members of the terpene synthase (TPS) gene family of higher plants. Surprisingly, several Arabidopsis terpene synthase-like sequences (AtTPS) share the most identity with TPS genes that participate in secondary metabolism in terpenoid-accumulating plant species. Expression of a putative Arabidopsis terpene synthase gene, designated AtTPS03, was demonstrated by amplification of a 392-bp cDNA fragment using primers designed to conserved regions of plant terpene synthases. Using the AtTPS03 fragment as a hybridization probe, a second AtTPS cDNA, designated AtTPS10, was isolated from a jasmonate-induced cDNA library. The partial AtTPS10 cDNA clone contained an open reading frame of 1665 bp encoding a protein of 555 amino acids. Functional expression of AtTPS10 in Escherichia coli yielded an active monoterpene synthase enzyme, which converted geranyl diphosphate (C(10)) into the acyclic monoterpenes beta-myrcene and (E)-beta-ocimene and small amounts of cyclic monoterpenes. Based on sequence relatedness, AtTPS10 was classified as a member of the TPSb subfamily of angiosperm monoterpene synthases. Sequence comparison of AtTPS10 with previously cloned monoterpene synthases suggests independent events of functional specialization of terpene synthases during the evolution of terpenoid secondary metabolism in gymnosperms and angiosperms. Functional characterization of the AtTPS10 gene was prompted by the availability of Arabidopsis genome sequences. Although Arabidoposis has not been reported to form terpenoid secondary metabolites, the unexpected expression of TPS genes belonging to the TPSb subfamily in this species strongly suggests that terpenoid secondary metabolism is active in the model system Arabidopsis.