The purpose of this work was to find an acoustic signal that will cause avoidance in free-swimming harbor porpoise without habituation and without alerting seals to the presence of gillnets filled with fish. If such a sound could be found, the next generation of pingers for gillnet attachment could be developed to reduce the incidental take of harbor porpoise.  In 1996, a total of 112 porpoises were observed during  32 days of sound trials using one type of sound, a 50 kHz ping that occurs at intervals comparable to commercially available pingers.  In 1996 weather conditions were vastly better than in 1993 (32 vs. 18 viable working days), but porpoise densities were at an all-time low for the region. These low numbers limited the number of valid trials available for testing, so only one sound type was tried during the seasons. This type of study can be very effective at determining the effects of acoustic signals on free-swimming marine mammals. However, better information on the hearing thresholds of porpoises at different frequencies is critically needed.  Otherwise, the uninformed fear that a very low probability event may injure a few individual animals will continue to impede the ability to answer the scientific questions necessary for the truly effective conservation and management of marine mammal populations.