P. WIGHTMAN, D.E. QUAIN AND P. G. MEADEN. 1996. Production brewing strains of the yeast Saccharomyces cerevisiae were analysed by DNA fingerprinting, using a Southern blotting and hybridization procedure and employing the Tyl-15 transposon as a probe. The ability to differentiate readily between strains was very dependent on the restriction enzyme used to digest the DNA prior to Southern blotting and hybridization; the enzymes EcoRI, PstI and SalI were found to be particularly useful in this respect. The method was applicable to the differentiation of both ale and lager yeasts, and was sufficiently sensitive to distinguish between very closely related strains. DNA fingerprinting by this approach confirmed, for example, that a flocculent strain isolated during a production-scale fermentation with a lager yeast was genotypically different from the parent.