Beta-dolabrin and gamma-thujaplicin isolated from Thujopsis dolabrata Sieb. et Zucc. var hondai Makino, like hinokitiol, showed strong phytogrowth-inhibitory activities, and their growth-inhibitory activities were as high as that of sodium 2,4-dichlorophenoxyacetate used as a positive control. In particular, the phytogrowth-inhibitory activity of gamma-thujaplicin was strong and it completely inhibited the germination of this seed of Brassica campestris L. subsp. rapa Hook f. et Anders at the concentration of 30 ppm. Both compounds exhibited inhibitory activities on B. campestris L. subsp. rapa Hook f. et Anders and Sesamum indicum Linne, even at the low concentration of 10 ppm. At 7 d after treatment with beta-dolabrin and gamma-thujaplicin, the amount of chlorophyll in the cotyledons of B. campestris L. subsp. rapa Hook f. et Anders treated with both compounds was greatly decreased as compared with the control. The findings indicate that the phytogrowth-inhibitory action might be a common biological activity of hinokitiol-related compounds, suggesting that at least a part of their phytogrowth-inhibitory actions seems to be related to a decrease in chlorophyll content.