U.S. Customs and Border Protection says the rounds were for a .22-calibre weapon and concealed beneath the front passenger seat of the vehicle's floor mat.
They say a 36-year-old man entering through a trusted traveller program lane last Wednesday had told authorities he had no items to declare but that a secondary inspection turned up the ammunition.
U.S. officials say he was questioned by Homeland Security and released.
They say the man also was issued a civil penalty and removed from the NEXUS program, which allows pre-screened travellers expedited processing by U.S. and Canadian officials.
Acting Port Director David Beculheimer says the program "allows low risk, frequent border travellers benefits not available to others."