Certain isolates of the bacterium Leuconostoc mesenteroides, such as strain NRRL B-1355, have been found to produce alternan, a polysaccharide with unique properties of potentially high commercial value. However, all of these isolates also produce significant amounts of the polysaccharide dextran, which would be costly to separate from alternan on a commercial basis. We developed a rapid screening method for the isolation of L. mesenteroides mutants that produce elevated proportions of alternan to dextran. With this technique, a set of mutants of strain NRRL B-1355 was isolated, including strain NRRL B-21138, which produced a high proportion of alternan to dextran and showed complete genetic stability after more than 60 generations.