Putative mutant set of the cv. Katy, a japonica rice well adapted for production in the southern U.S. Katy (PI 527707) has resistance to sheath blight disease (Rhizoctonia solani) and possesses the major blast resistance genes Pi-ta, Pi-ta2, and Pi-ks. The first subset of 15,192 putative mutants were treated by fast neutrons with 7.7 Gy, 26.3 Gy, and 49.4 Gy. A total of 15,192 M1 panicles were harvested from 15,192 M0 and were subsequently amplified to M4 by single seed breeding method in Stuttgart, AR, from 2003 to 2006. An evaluation of 192 randomly selected plants from the entire population in 2006 verified that they were derived from Katy based on 10 diagnostics simple sequence repeat markers. (Access the results of this evaluation on the Collection Catalog page of the GSOR Homepage.) This population has been used to identify blast susceptible mutants that can be used to study molecular mechanisms of disease resistance (Jia and Martin, 2008). This population will be useful for the identification of rice mutants that correlate with any gene in the rice genome.