Mexican police say they have seized more than 250,000 rounds of ammunition that were being smuggled into the country from the US.
The ammunition for assault rifles was found on a trailer driven across the border from Texas into Ciudad Juarez.
The driver - a 37-year-old man from Dallas, Texas - was detained.
Mexico says many of the weapons used by violent drugs gangs come from the US. It wants tighter controls on weapons sales north of the border.
The driver told customs officials he had nothing to declare when he arrived at the border, Mexican prosecutors said.
But an x-ray inspection of his cargo hold revealed the metal boxes holding the ammunition.
In total 268,000 bullets for AK-47 and AR-15 assault rifles were seized, Ciudad Juarez border control official Juan Ramon Huerta said.
Mexican President Felipe Calderon has repeatedly called for the US to implement stricter firearms laws to cut the supply of weapons to drugs cartels.
Around 50,000 people have died in drug-related violence in Mexico since President Calderon began deploying troops to fight the cartels in 2006.