The gorgonian Plexaurella grisea from Punta Cana, Dominican Republic, contains five new acyclic sesquiterpenes, (3E,5E)-3,7,11-trimethyl-9-oxododeca-1,3,5-triene (3), (3Z,5E)-3,7,11-trimethyl-9-oxododeca-1,3,5-triene (4), (3E)-6-acetoxy-3,11-dimethyl-7-methylidendodeca-1,3,10-triene (5), (3E,5E)-7-hydroxy-3,7,11-trimethyldodeca-1,3,5,10-tetraene (6), and (3E,5E,9E)-8,11-diacetoxy-3,7,11-trimethyldodeca-1,3,5,9-tetraene (7), and two new linear norsesquiterpenes, (2E,4E,7Z)-2,6,10-trimethylundeca-2,4,7,9-tetraenal (8) and (2E,4E)-2,6,10-trimethylundeca-2,4,9-trienal (9), in addition to the known compounds 1, 2, and 10. The structures of the new compounds 3-9 were elucidated by interpretation of spectroscopic data. In general, the new compounds are mildly cytotoxic against tumor cell lines, and although norsesquiterpene 8 was inactive, norsesquiterpene 9 exhibited the greatest and selective activity against the P-388 tumor cell line.