Periconicin A and B, two new fusicoccane diterpenes originally isolated from the cultures of endophytic fungus Periconia sp. OBW-15, were tested by several biological assays. Periconicin A was consistently more active than periconicin B. In an antifungal activity assay, periconicin A showed potent inhibitory activity against the agents of human mycoses, including Candida albicans, Trichophyton mentagrophytes, and T. rubrum, with minimum inhibitory concentration (MIC) in the range of 3.12- 6.25 $mug$ /ml. In a plant growth regulatory activity assay, periconicins inhibited hypocotyl elongation and root growth of Brassica campestris L. and Raphanus sativus L. At concentrations below 1 μg/ml, however, both compounds accelerated root growth by 110- 135%. From these results, it is apparent that a methyl group positioned in a cyclopentane ring may play an important role in plant and fungal growth inhibitory activity.