In August, 1806, Argüello succeeded his father as Commandant of California with the rank of lieutenant. In 1821, he launched an exploration of Northern California to investigate the rumors of "Foreigners" setting up a base. Argüello published his expedition diary, titled "The Diary of Captain Arguello: The Last Spanish Expedition in California, October 17 - November 17, 1821". He ordered retribution on hunters of the Russian-American Company who were catching sea otters in San Francisco Bay. There were numerous sea otter populations to hunt and a lack of Spanish military posts above San Francisco Bay made it hard for Argüello to stop this northern activity and the possibility of them trading with the Spanish Missions.