In June 1964, "Grouper" was awarded the Battle Efficiency "E". In November 1965, the submarine again entered the Philadelphia Naval Shipyard for overhaul and equipment modifications to increase her usefulness as a floating underwater sound laboratory. She departed Philadelphia on 1 May 1966, reached New London on 1 June, and headed for the Caribbean for intensive research. Her studies during 1966 also took her to Narragansett Bay and twice to Bermuda. At the beginning of 1967, "Grouper" was at New London preparing to resume gathering knowledge of underwater sound propagation.