Rice produces a number of phytoalexins, and at least one allelopathic agent, from syn-copalyl diphosphate (CPP), representing the only known metabolic fate for this compound. Thus, the class II terpene synthase that converts the universal diterpenoid precursor geranylgeranyl diphosphate to syn-CPP catalyzes the committed step in biosynthesis of these natural products. Here the extensive sequence information available for rice was coupled to recombinant expression and functional analysis to identify syn-copalyl diphosphate synthase (OsCPSsyn). In addition, OsCPSsyn mRNA was found to be specifically induced in leaves by conditions that stimulate phytoalexin biosynthesis. Therefore, transcription of OsCPSsyn seems to be an important regulatory point for controlling the production of these defensive compounds. Finally, alignments carried out with OsCPSsyn revealed that class II terpene synthases exhibit a sequence conservation pattern substantially different from that of the prototypical class I enzymes. One particularly notable feature is the specific conservation of the functionally cryptic ‘insertional’ sequence element in class II terpene synthases, indicating that this region is important for the corresponding cyclization reaction.