Geosmin (trans-1,10-dimethyl-trans-9-decalol), an earthy-musty compound, has been identified in wines and in grape juice, in which its presence is highly detrimental to the aromatic quality. Geosmin has a biological origin, and the analysis of rotten grape microflora has been done on two grape varieties (Semillon, Cabernet Sauvignon) from six parcels of the Bordeaux region over 3 years (1999, 2000, 2001). Forty-three Penicillium-related species have been analyzed by gas chromatography-mass spectrometry (GC-MS) for their geosmin production. GC-MS analysis has demonstrated that the earthy odor was always correlated with the presence of geosmin. Phenotypic characterization of Penicillium spp. being ambiguous, a molecular characterization by rDNA internal transcribed spacer (ITS1) sequencing was performed for all strains. The results evidenced that all strains producing geosmin belonged to only one species, P. expansum, and that the other strains, not producing geosmin, belonged to three species: P. purpurogenum, P. thomii, and Talaromyces wortmanii.