The way compatible pairs of nitrate non-utilizing mutants (nit) are selected is usually not explained and remains unclear whether these pairs are representative for an isolate or strain. In addition, tester strains of Verticillium dahliae vegetative compatibility groups (VCGs) cross-react with at least one pair of tester strains of another VCG, and although it is a common knowledge of scientists working with the fungus that reversion of nit mutants to wild type occurs far too often, this fact is rarely mentioned in papers. To overcome the above problems, a protocol was developed for the generation of large number of nit mutants from any given isolate and to ensure that compatible pairs of mutants are indeed stable and the most frequent within the putative tester mutants produced from each isolate. Thus, we provide a reproducible and objective way of selecting V. dahliae tester strains for each isolate and VCG. Although VC grouping is based on the formation of stable heterokaryons, we demonstrate in this work that cross-reactions cannot be eliminated and that strict genetic barriers between two main VCG groups are absent in V. dahliae.