The Interior Ministry says the density gauge was found yesterday evening in Ciudad Obregon after somebody told authorities a driver had left a black bag on a sidewalk beside a stop sign.
It had been stolen from a Sonora state road works pickup truck in the same city a day earlier.
Today's statement from the ministry says the device contained cesium, americium and beryllium. Such gauges are commonly used in road construction.
There have been a half-dozen thefts of radioactive material in Mexico in the last two years. The hazardous materials have been recovered in the previous cases.