The carbon-monoxide (630080) (CO) concentration in the cabs of aircraft fueling vehicles was studied. Seventeen general air samples were collected from cabs with windows closed and the heater on and the vehicle positioned in several orientations to the wind. Seven personal samples were also taken from fuelmen. CO concentrations for the personal samples ranged from 5.5 to 14.5 parts per million (ppm), and the CO concentration in the cabs ranged from 5 to 300ppm. With the vehicle facing into the wind, away from the wind, parked perpendicular to the wind, out of the wind with heater off, and out of the wind with window open, the mean CO concentrations in the cabs were 22, 300, 150, 300, and 160ppm, respectively. About 50 percent of the aircraft fueling vehicles surveyed had CO concentrations well above the 35ppm recommended 8 hour time weighted average. The authors recommend improved vehicle maintenance to eliminate holes in vehicle mufflers, floor boards, and around pedals, and engineering controls to eliminate CO as an occupational hazard to fuelmen.