Thirty-three wild-type strains of Verticillium dahliae were isolated from potato stems from 25 fields in southern Idaho. These strains were all assigned to vegetative compatibility group (VCG) 4 based on pairings of nitrate-nonutilizing mutants induced on medium containing chlorate. The Idaho strains and VCG 4 testers were divided into nine subgroups. These subgroups relate to previously designated VCG 4 subgroups as follows: 4A = 4A1, 4A2, 4A3, 4A4, 4A5, and 4A6; 4A/B = 4A/B1; and 4B = 4B1 and 4B2. Subgroup 4A contained 29 of the 33 Idaho strains. All subgroup 4A strains and most of the strains from subgroups 4A/B and 4B were pathogenic in greenhouse studies. When placed into three subgroups for analysis, the strains from subgroup 4A were significantly more virulent than those from subgroups 4A/B and 4B. However, not all individual subgroup 4A strains were significantly more virulent than strains in subgroups 4A/B and 4B. This continuum of virulence and genetic diversity among V. dahliae strains was evenly distributed throughout potato-growing areas in southern Idaho.