Two collections of strains of Leucothrix mucor were isolated, one of world-wide origin and the other originating from a restricted area around San Juan Island, Friday Harbor, Washington. These two collections were compared on the basis of morphological, nutritional, and physiological properties, deoxyribonucleic acid base composition, and mobility by starch-gel electrophoresis of isocitric dehydrogenase and malic dehydrogenase. Despite the fact that all the strains were similar in most characteristics, significant molecular heterogeneity was found in the enzyme electrophoresis studies. This heterogeneity was just as great in the local collection as it was in the world collection. Even strains originating from the same microenvironment exhibited molecular heterogeneity.