<p>A new sterol, langcosterol A (<b>1</b>), together with two known sterols <b>2</b> and <b>3</b>, were isolated from the marine sponge <i>Xestospongia testudinaria</i> collected in Vietnam. Their chemical structures were elucidated on the basis of extensive spectroscopic analyses and comparisons with published data. The new compound <b>1</b> and the known compound <b>3</b> exhibited moderate cytotoxic activities against three human cancer cell lines (A549, lung cancer; MCF7, breast cancer; HeLa, cervical cancer) and a human normal cell line (WI-38 fibroblast), with IC<sub>50</sub> values ranging from 29.0 to 68.0 μM.</p
