Thousands of porpoises die annually in monofilament gillnets. Simple net modifications may alleviate the problem. In June through August 1988, we quantified harbour porpoise (Phocoena phocoena) reactions to weighted vertical lines suspended from corklines. If effective in deterring porpoises, vertical lines might be threaded into gillnets to reduce entanglement rates. No structure deflected all porpoise groups, but strands consisting of small metallic spheres and surgical tubing turned a greater proportion of porpoises than did polypropylene line verticals. Although the structures may not be useful in reducing cetacean mortality in gillnets, the experiment may have merit in identifying areas for continued research in attempts to reduce cetacean entanglement in gillnets.