The title diterpenoid compound, (I), which is known as entkaur-16-ene-13,19-diol (Subrahmanyan et al., 1999; Han et al., 2004), was isolated from the roots of Bruguiera cylindrica (Rhizophoraceae), a mangrove plant which is found in southeast Asia. In our continuing research on bioactive compounds from mangrove sources (Chantrapromma et al., 2006; Chumkaew et al., 2006; Fun, Chantrapromma et al., 2006; Fun, Pakhathirathien et al., 2006), we have studied the chemical constituents from the roots of Bruguiera cylindrica (Rhizophoraceae). Our studies show that the CH2Cl2 extract of the roots of this plant shows strong cytotoxic activity against MCF-7 (breast cancer) and HT-29 (colon cancer) cell lines. Previously, we have reported the isolation and crystal structure of another compound, 3--feruloyltaraxerol, from this plant (Chantrapromma et al., 2003). We report here the crystal structure of (I). The title compound crystallizes with two independent molecules, A and B, in the asymmetric unit. The two molecules have similar bond lengths but the O—C—C angles at C13 are slightly different (Table 1). The bond distances and angles are within normal ranges (Allen et al., 1987). The molecule of (I) contains a fused four-ring system A/B/C/D (Fig. 1). The A/B ring junction is trans-fused, and B/C and C/D are cis-fused. In both molecules, the cyclohexane rings adopt chair conformations and the cyclopentane ring has an envelope conformation, with atom C14 displaced from the C9/C13/C15/C16 plane by 0.301 (2) A˚ in molecule A and 0.299 (2) A˚ in molecule B; the Cremer & Pople (1975) puckering parameters Q and are 0.465 (2) A˚ and 30.4 (3), respectively, in molecule A, and 0.464 (2) A˚ and 29.9 (3) in molecule B. The hydroxyl group is equatorially attached at atom C13. The methylene group is equatorially attached to the cyclopentane ring at atom C16. The hydroxymethyl group is axially attached to the cyclohexane ring A at atom C5. The orientation of the hydroxymethyl group is described by the C4—C5—C19—O1 torsion angle, which is 65.0 (2) in molecule A [67.2 (2) in molecule B]. The crystal packing of (I) is stabilized by intermolecular O—H---O hydrogen bonds (Table 2). The molecules are linked into a two-dimensional network parallel to the bc plane (Fig. 2).