In the course of search for potent chitin synthase inhibitors from plant extracts, the chitin synthase 2 inhibitors, O-methyl pisiferic acid and 8,20-dihydroxy-9(11),13-abietadien-12-one which have diterpene skeleton, were isolated from the leaves of Chamaecyparis pisifera. These compounds inhibited chitin synthase 2 of Saccharomyces cerevisiae with the IC₅₀ values of 5.8 and 226.4 μM, respectively. Especially, O-methyl pisiferic acid showed 15.3-fold stronger inhibitory activity than polyoxin D (IC₅₀=88.6 μM), a well-known chitin synthase inhibitor. These compounds exhibited weaker inhibitory activities against chitin synthase 1 than chitin synthase 2, whereas it showed no inhibitory activity for chitin synthase 3. The compound exhibited mixed competitive inhibition with respect to UDP-N-acetyl-D-glucosamine as substrate (K_i=5 μM). These results indicated that O-methyl pisiferic acid is a specific inhibitor of chitin synthase 2. The compound also inhibited chitin synthase 1 of Candida albicans, which represents analogues to chitin synthase 2 of S. cerevisiae, with an IC₅₀ of 75.6 μM, which represents 1.8-fold weaker activity than that of polyoxin D. Although O-methyl pisiferic acid has been reported for antibacterial and insecticidal activities, the present study is the first report on its inhibitory activity against chitin synthase 2.