Leptosins O (1), P (2), Q (3), R (4) and S (5) have been isolated from a strain of Leptosphaeria sp. originally separated from the marine alga Sargassum tortile. Their absolute stereostructures have been elucidated on the basis of spectroscopic analyses of their acetate derivatives (6–10) using various 1D and 2D NMR techniques and some chemical transformations. The NMR and NOE spectral analyses of 6–10 revealed that they exist in a single conformer of B type in CDCl3. Among these metabolites, leptosins O (1) and P (2) exhibited significant cytotoxicity against cultured P388 cells