Crop coefficient values for drip irrigated cotton grown near Casa Grande, Arizona in 1982 were obtained from estimates of actual and potential evapotranspiration using measured amounts of irrigation and rainfall, changes in soil-water content, and micrometeorological measurements. Potential evapotranspiration was estimated from the Penman formula using primarily environmental data collected near the cotton on a hourly basis. Semi-monthly crop coefficient values ranged from 0.03 in early April to 0.93 in late August. Seasonal cotton evapotranspiration was estimated to be 31.3 inches and seasonal potential evapotranspiration was estimated to be 67.4 inches, resulting in a seasonal crop coefficient value of 0.46.