Four new bis-beta-carboline alkaloids, quassidines A-D (1-4), together with a known alkaloid, picrasidine C (5), were isolated from the stems of Picrasma quassioides. Quassidine A (1) is the first reported bis-beta-carboline alkaloid possessing a novel cyclobutane moiety. The structures of the new compounds were determined on the basis of their 1D and 2D NMR and X-ray diffraction data. A possible biogenetic pathway for these alkaloids was proposed, and all compounds were evaluated for anti-inflammatory activity. Only quassidine A (1) showed weak activity.