A total of 105 seed samples collected from commercially available potato seed lots in Turkey were assayed for Verticillium dahliae. V. dahliae was successfully isolated from 29 of the 105 seed lots tested. The frequency of infected lots was nearly 28%. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. Of the 110 isolates obtained, 63 were assigned to VCG4A, 24 to VCG4B, 19 to VCG2A and one to VCG2B, while the three remaining isolates could not be assigned to any of the identified VCGs. All 36 of the isolates tested in the greenhouse on potato cv. 'Russet Burbank' were pathogenic to potato. As a group, AUDPC values were significantly higher (P < 0.05) for VCG4A than for VCG4B and VCG2 isolates. These data suggest that (i) commercial potato seed lots are commonly infected with V. dahliae, and that this is a primary method by which the pathogen can be introduced into production fields; (ii) potato isolates of V. dahliae belong to VCG4A, 4B and 2A and these isolates are widely distributed via seed lots; and (iii) VCG4A and VCG4B are distinct pathotypes of V. dahliae that vary in their aggressiveness to potato. The present study is the first report of natural infections of potato by VCG4A and VCG2A in Turkey.