||Breeding for resistance to rust diseases in wheat is an example of productivity maintenance research. Productivity maintenance research is necessary to avoid contractions in the wheat supply curve that result from changes in the biological or physical environment. In this study, the benefits of incorporating nonspecific resistance to leaf rust caused by Puccinia recondita into modern bread wheats (Triticum aestivum) have been estimated using data on resistance genes identified in cultivars, trial data, and area sown to cltivar in the Yaqui Valley, Sonora State, Mexico. In the most pessimistic scenario, the gross benefits generated in the Yaqui Valley from 1970 to 1990 were 17 million U.S. dollars (in 1994 real terms). Even when costs were overstated and benefits were understated, the internal rate of return on capital invested was 13%, well within the range recommended for use in project evaluation by the World Bank. Substantial economic benefits likely are associated with development of nonspecific resistance in many wheat-producing areas of developing countries where farmers change cultivars slowly because of delays in cultivar release, incomplete seed markets, and economic factors related to adoption or where disease pressure is heavy and the costs of treating disease outbreaks is high.