The stereochemical course of GGDP cyclization in biosynthesis of phomactins is proposed by the stereochemistry of a cyclization product, phomacta-1(14) ,3,7-triene, isolated from Phoma sp. and the results of incorporation experiments with [1-¹³C]- and [1,2-¹³C] acetates. This stereospecific rearrangement suggests that the cyclization of GGDP toward verticillen-12-yl cation proceeds through a boat-form transition state. Isolation of series of phomactatriene derivatives from mycelial extracts of the phomactin-producing fungus indicated involvement of verticilen-12-yl cation. Quantum mechanic methods (MOPAC 6.0) proposed involvement of the related cationic intermediates, verticilen-11-yl, 8-yl and 4-yl cations which would afford phomactatriene, taxadiene and verticillatriene, respectively. To elucidate a detailed reaction mechanism of macrocyclic formation by diterpene synthase, non-enzymatic reaction of verticilen-12-yl cation was conducted. Treatment of verticillol with BF₃·(OEt₂) afforded phomacta-1(15),3,7- triene and taxadiene. These experimental results provide further evidence on involvement of three cationic intermediates which are predicted by MOPAC calculation.