Bioactivity-guided fractionation of a MeOH extract of the stem bark of Cyathocalyx zeylanica afforded a moderately bioactive new azafluorenone alkaloid, cyathocaline, the structure of which was established as 5,7-dihydroxy-6-methoxy-1-methyl-4-azafiuoren-9-one [1] with the aid of spectroscopic data. As a part of our continuing efforts to discover molecules of biological interest from the Sri Lankan flora, we have initiated the screening of Sri Lankan plants in the family Annonaceae for potential cytotoxic compounds employing the brine shrimp toxicity assay (1). An extract of Cyathocalyx zeylanica Champ. ex Hook. f. & Thorns. showing a positive response to this bioassay was selected for further investigation. In Sri Lanka, the genus Cyathocalyx is represented by a single species, C. zeylanica, which is endemic to the country (2). There is no reported work on this genus. A MeOH extract of C. zeylanica, which showed a positive response to the brine shrimp assay (3), was also found to be active in our mechanism-based yeast bioassay for DNA-modifying agents (4). Bioactivity-guided fractionation utilizing the latter bioassay resulted in the isolation of a moderately bioactive new azafluorenone alkaloid, named cyathocaline. In this paper we present the structure elucidation and biological activity of cyathocaline [1].