One new 5-acetoxy-2,10-dibromo-3-chloro-7,8-epoxy-alpha-chamigrene (1) and three known sequiterpenes, namely 2, 10-dibromo-3-chloro-7-chamigrene (2), deoxyprepacifenol (3), and pacifenol (4), were isolated from both the sea hare Aplysia parvula and the red seaweed Laurencia filiformis. On the other hand, pentadecanal and its aldol product (E)-2-tridecyl-2-heptadec-2-enal (6) were only separated from L. filiformis while the previously known fimbrolide (5) from Delisea sp. and the known purple pigment, aplysioviolin (7), were isolated only from the sea hare A. parvula. The structure of aplysioviolin (7) has been revised and the nuclear magnetic resonance spectra of (2) fully assigned for the first time. A single crystal X-ray study is reported for (1). The brine shrimp bioassay showed 90% mortality using pacifenol, at a concentration of 23 mug/mL in seawater after 24 h as the strongest activity among all the test compounds (1)-(4) and (7).