During the search for natural antibiotics from fungal metabolites, a new cyathane diterpenoid, fragilicine A (1), and three known cyathane diterpenoids, erinacines I, A, and B (2–4) were isolated from the culture broth of Dentipellis fragilis. Chemical structures of 1–4 were determined by analyses of 1D- and 2D-NMR and MS data and by comparisons with data of those reported in the literature. These isolated compounds were assessed for their antimicrobial activities against Bacillus subtilis, B. atrophaeus, B. cereus, Listeria monocytogenes, Fusarium oxysporum, Diaporthe sp., and Rhizoctonia solani. These compounds showed weak antimicrobial activities.