Recurrent phenotypic selection, a form of mass selection, proved effective for developing resistance in two populations of alfalfa (Medicago sativa L.) to the following (i) diseases: rust, common leafspot, bacterial wilt, and anthracnose, caused by Uromyces striatus Schroet. var. medicaginis (Pass.) Arth., Pseudopeziza medicaginis (Lib.) Sacc., Corynebacterium insidiosum (McCull.) H. L. Jens., and Colletotrichum trifolii Bain and Essary, respectively; and (ii) insect pests: spotted alfalfa aphid [Therioaphis maculata (Buckton)] and potato leafhopper [Empoasca fabae (Harris)]. general vigor of the populations also increased in generations of selection conducted in the field. The success of mass selection in alfalfa prompted us to suggest ways in which this breeding method could be used to conserve genetic diversity still available in alfalfa, develop combined resistance to disease and insect pests, and reduce dependence on pesticides. We propose an integrated program of worldwide collection, recombination, and mild selection to conserve and improve alfalfa germplasm resources. The key feature of the program is the development of seven gene pools by mass selection, representing seven geographic regions and a wide range of environments, to provide improved source material for breeding regionally adapted, multiple pest-resistant varieties. Resistant varieties developed by recurrent mass selection can be expected to be less vulnerable to attack by mutant forms of the pathogen or insect than varieties developed by breeding procedures which give more attention to genetic uniformity.