Aims: The beta-glucosidase activity is involved in the hydrolysis of several important compounds for the development of varietal wine flavour. The aim of the present study was to investigate the production of beta-glucosidase in a number of wine-related yeast strains and to measure and identify this activity over the course of grape juice fermentation. Methods and Results: beta-glucosidase activity was measured as the amount of 4-methylumbelliferone released from 4-methylumbelliferyl-beta-D-glucopyranoside substrate. Intact cells of some grape and wine-spoilage yeasts showed beta-glucosidase activity much higher than those observed in wine yeasts 'sensu stricto'. During fermentation, three Saccharomyces cerevisiae strains, one Hanseniaspora valbyensts strain and one Brettanomyces anomalus strain showed fl-glucosidase activity both intra- and extracellularly. Conclusions: In the studied strains, beta-glucosidase activity was at its maximum when the cells were in the active growth phase. However, a lowering of medium pH to values around 3 during fermentation led to total loss of activity. Significance and Impact of the Study: During the course of this study, a new, rapid and reproducible method to assay beta-glucosidase activity was developed. The fact that Saccharomyces and non-Saccharomyces yeast strains are able to express beta-glucosidase activity during the alcoholic fermentation sheds new light on the contribution of these yeasts in the aroma expression of wines.