THE TITLE ACRIDONE ALKALOID [SYSTEMATIC NAME: 1,3,5-trihydr-oxy-2,4-bis-(3-methyl-but-2-en-yl)acridin-9(10H)-one], C(23)H(25)NO(4), known as atalaphylline, was isolated from Atalantia monophylla Corrêa, a mangrove plant. The mol-ecule contains three fused planar rings with an r.m.s. deviation of 0.026 (2) Å. Both 3-methyl-but-2-enyl substituents are in a (-)anticlinal conformation. An intra-molecular N-H⋯O hydrogen bond generates an S(5) ring motif, while an intra-molecular O-H⋯O hydrogen bond generates an S(6) ring motif. In the crystal structure, the mol-ecules are linked into screw chains along [010] by inter-molecular O-H⋯O hydrogen bonds. These chains are stacked along the a axis by π-π inter-actions with centroid-centroid distances of 3.6695 (13) and 3.6696 (13) Å.