On 19 September 1944, "Cero" cleared Darwin, Australia, for the Mindanao and Sulu Seas for her sixth patrol. She called "en route" at Mios Woendi, where she took on board of supplies for Philippine guerrillas, along with 16 soldiers headed for behind-the-lines operations in Luzon. Although not permitted by her orders to attack escorted merchantmen while on this mission, "Cero" encountered two small craft on 27 October, and in a resulting gun action, damaged both and forced them ashore. On 3 November, north of Manila, she made contact with the guerrillas, landed the soldiers and supplies, and took aboard four evacuees. Later attacked by a Japanese submarine, an alert bridge crew, led by Jim "Red Dog" Vermillion, enabled "Cero" to evade a torpedo aimed at her. Mission completed, she returned to Pearl Harbor 24 November, then sailed to the West Coast for overhaul.