Two new cytotoxic cytochalasins were isolated by brine shrimp bioassay-guided fractionation from a culture of the wood inhabiting fungus, Xylaria obovata. Their structures were determined as 19,20-epoxycytochalasin Q (1) and its deacetyl analog 2 by the application of spectroscopic techniques and chemical correlation with cytochalasin R. Acetylation of 2 yielded 1. Both 1 and 2 were cytotoxic but were found to be inactive in an HIV-protease inhibitory assay and a mechanism-based DNA damaging yeast assay.