Brettanomyces sp. and its ascosporogenous sexual state, Dekkera sp., have been well documented as spoilage microorganisms, usually associated with barrel-aged red wines. In this report, we describe the genetic characterization, on the basis of DNA content per cell, electrophoretic karyotyping, and mitochondrial DNA restriction patterns, of a Dekkera yeast strain isolated from sherries and of a number of other Brettanomyces and Dekkera strains. By using a genomic DNA fragment of the isolated Dekkera strain, we developed a two-step PCR method which directs the specific amplification of target DNA from this strain and from other Brettanomyces-Dekkera strains. The method efficiently amplified the target DNA from intact cells, obviating DNA isolation, and yielded a detection limit of fewer than 10 yeast cells in contaminated samples of sherry.