A cytotoxic sesquiterpene against L1210 cell, named pubetalin. was isolated from the herb of Siegesbeckia pubescens Makino. Its structure was identified as $6-formyl-2,;3,;3{alpha},;4,;5,;8,;9,;11{alpha}-octahydro-10-hydroxymethyl-5-methoxy-3-methylene-2-oxocyclodeca[{eta}];furan-4-ylester$ of 2-methyl-2-propenoic acid.