The LEU2 gene coding for beta-isopropylmalate dehydrogenase of the yeast Kluyveromyces lactis strain AWJ137 was disrupted. In the resulting Leu(-) strain a 0.57x10(3)-base pairs PstI/BglII fragment of the LEU2 coding region was replaced by the TRP1 gene of Saccharomyces cerevisiae. The mutant strain was characterized by stability tests and a physical map of the disrupted region was established by restriction-enzyme analysis combined with hybridization experiments. The usefulness of the mutant strain as a recipient was shown by transformation experiments.