Economically and environmentally sound crop production entails, among other things, efficient use of N inputs. This research was conducted to determine the response of irrigated Acala cotton to different N fertilization rates, and to track applied ¹⁵N fertilizer over time. Cotton was grown at two locations in the San Joaquin Valley (CA), on a Typic Haplocambid (clay loam) and a Typic Torriorthent (sandy loam). At both locations N treatments of 50, 100, 150, and 200 lb acre^-1 were established as a randomized complete block design with four replications. Microplots established in the 50 and the 150 lb acre^-1 treatments were fertilized with ¹⁵N Urea. Response of lint yield to N treatment was absent or minimal at both locations. However, total biomass accumulation increased with increasing N application rates. The absence of a strong yield response indicates a high supply power of the soil from mineral soil N in the spring and N mineralization during the growing season.