ABSTRACTPrevious studies of isozyme banding patterns inUromyces appendiculatuscompared one isolate per population or location. In this study, five or more isolates were surveyed from each of 12 field collections to estimate genetic variation within and between the source populations. Five enzymes stained in Polyacrylamide slab gels produced 10–21 bands grouped as 10 markers (putative loci); 14 of these bands were present in>72% of the isolates. Six markers indicated heterozygous loci. Marker I of phosphoglucomutase was the most diverse across the collections; three markers were fixed. One marker pattern was predominant for each enzyme, and one multi-isozyme phenotype occurred in 11 of the 12 collections and 21 of the 66 isolates. A 1986 collection from west-central Minnesota, denoted P24, had the lowest Shannon Diversity Index, the highest proportion of simple (homozygote-like) band patterns, the highest mean number of marker differences from isolates of the other collections, and no phenotypes in common with any other collection. Nei's Coefficient of Genetic Identity averaged 0.952 among 11 of the collections but only 0.791 between P24 and the other collections. Collection P24 may represent a late-season epidemic population of asexually produced spores.