A project was initiated in the 2004 cotton (Gossypium hirsutum) growing season in an effort to accomplish two major objectives. The first was to continue with phosphorus (P) fertilization evaluations that have taken place in the Upper Gila River Valley for the past four seasons by continuing to document the effects of P fertilization on crop yield and fiber quality and to validate the 5 ppm soil P critical level. The second objective of the project was to investigate the feasibility of utilizing Global Positioning Systems (GPS) and Geographic Information Systems (GIS) technology to perform variable rate (VR) applications of P fertilizers based upon yield data from the previous cropping season. Yield maps from a yield monitoring system were collected from the 2003 growing season and used to develop prescription applications of P fertilizers in the 2004 season. The project was established with four treatments including a control (0 P fertilizer applied); a VR application treatment (58 kg P/ha - average); a high Uniform Rate (UR) application (84 kg P/ha); and a low UR application (50 kg P/ha). These treatments were arranged in a randomized complete block design with four replications. Lint yield results indicated a positive response to applied P fertilizers with treatments receiving P producing significantly higher yields than the control. Lint yield was not significantly affected by fertilizer application technique. Yield differences among the treatments receiving P fertilizer were minimal. However, the amount of P fertilizer used in the VR treatment was reduced by 27% with the use of the VR application technique. This produces an approximate $17/ha savings to the grower. Results also indicate the need for growers to consider P fertilization as an important component of their fertility management program in soils where soil P levels are below 5 ppm.