A new quinolone, cyclomegistine, was isolated from the bark of Sarcomelicope megistophylla. This alkaloid possesses the cyclobuta[b]quinoline ring system that has not been previously described either from natural or from synthetic origin. Biogenetically, cyclomegistine could arise from the oxidative aromatic ring cleavage of an acridone precursor, followed by photoisomerization of the resulting butadiene into the isomeric cyclobutene. © 2001 Elsevier Science Ltd.