Late in June she carried heavy equipment for the Inter-Oceanic Canal Study Group. After overhaul in the spring of 1967 begun at Maryland Shipbuilding & Drydock Company of Baltimore, and completed at Jacksonville Shipyards, Jacksonville, Florida the LST returned to Little Creek, Virginia on 19 June for training. She got underway on 25 August for another deployment with the Canal Zone Amphibious Group. During this deployment she transited the canal on four occasions for operations along the Pacific coast of Central and South America. After returning to Little Creek on 17 December, the tank landing ship resumed training operations. Throughout 1968 "Middlesex County" conducted local operations in the Atlantic and Caribbean, continuing these operations into 1969.