A simulation model of growth and development for cotton was developed and integrated in the Decision Support System for Agrotechnology Transfer (DSSAT). The new cotton model was developed using the biophysical framework provided by CSM-CROPGRO. The objective of the study was to evaluate the new cotton model for farmers' field conditions and for a research site in southwest Georgia. During the 2004 growing season one cotton field in Baker County and two cotton fields in Mitchell County were monitored from mid-April until the beginning of November. Plant samples for growth analysis, leaf area index (LAI), and canopy height were collected every two weeks. The observed irrigation amounts were obtained from the database of the Agricultural Water Pumping (AWP; www.AgWaterPumping.net) program. For the precipitation data, a rain gauge equipped with a HOBO Event Data Logger was installed in each farmer's field. Air temperature and solar radiation data were collected from the nearest automated weather station to each site (www.Georgiaweather.net). Concurrent with field monitoring, an experiment was conducted at the Stripling Irrigation Research Park in Camilla, Mitchell County. Data on cotton growth and development under irrigated and rainfed conditions were collected every two weeks. For all sites, the model simulated dry matter accumulation for leaves and stems up to the early reproductive growth fairly well. For irrigated conditions, the model simulated biomass accumulation for bolls up to about 120 days after sowing in good agreement with observed data. The model generally underestimated total boll biomass for rainfed conditions. Further model evaluation is planned for the 2005 growing season.