Investigation of the EtOAc extract from the Thai sea hare, Bursatella leachii, resulted in the isolation of a potent stimulator of actin assembly, hectochlorin (1), and its new derivative, deacetylhectochlorin (2). Compound 2 exhibited more potent cytotoxicity than 1 against different human carcinoma cell lines. In addition, a new morpholine-2,5-dione analogue, syn-3-isopropyl-6-(4-methoxybenzyl)-4-methylmorpholine-2,5-dione (3), was co-isolated.