Hotter years are associated with lower economic output across countries. This paper uses high-frequency micro data from selected firms in India and finds individual worker productivity on hot days declines by 2 to 4 percent per degree Celsius. Sustained heat also increases worker absenteeism. Using a national panel of manufacturing plants, we find similar temperature effects on output and show that these are driven by reductions in the productivity of labor. Our estimated effects on individuals are sufficient to explain output declines in plants and these effect sizes are in turn consistent with those in studies that rely on country GDP panels.