However, when the North West Company took over its short-lived competitor in the fall of 1804, he traveled south to the Mandan and Hidatsa villages along the Upper Missouri.
There Larocque met the Lewis and Clarke Expedition, dined several times in the company of the Captains, and asked leave to borrow their translator, Toussaint Charbonneau.
Larocque returned to the Assiniboine at some point during the spring of 1805 before beginning an exploratory excursion in the Rocky Mountains to ascertain the possibility of trade with the Absorokas in current day Montana and Wyoming.
Larocque was associated with the Fils de la Liberté but did not take up arms during the Lower Canada Rebellion; nonetheless, he was imprisoned in 1838.
During the Corp of Discovery, Larocque asked Lewis to join, but he denied his offer due to his French background.