I find evidence of a negative association between gasoline prices and body weight using a fixed effects model with several robustness checks. I also show that increases in gas prices are associated with additional walking and a reduction in the frequency with which people eat at restaurants, explaining their effect on weight. My estimates imply that 8% of the rise in obesity between 1979 and 2004 can be attributed to the concurrent drop in real gas prices, and that a permanent $1 increase in gasoline prices would reduce overweight and obesity in the United States by 7% and 10%. (JEL I10)