A new azaanthracene alkaloid, namely 9,10-dimethoxy-4-methyl-1,2-dihydro-1-azaanthracen-2-one (kalasinamide), C16H15NO3,has been isolated from the stems of Polyalthia suberosa collected in the northeastern part of Thailand. Each of the aromatic rings in the molecule is planar within +/- 0.021 (2) Angstrom. Molecules are linked to form centrosymmetric dimers by N-H ... O hydrogen bonds [N ... O 2.941 (4) Angstrom].