Of 60 different thermophilic enrichment cultures, 16 converted glycerol anaerobically to 1,3-propanediol. Two PD-forming strains were further enriched, isolated, and characterised. For the most active strain, AT1, the optimal cultivation parameters for pH and temperature were determined as 5.8 to 6.0 and 58 degreesC, respectively. In batch-fermentations with AT1, 6.4 g propanediol per litre was formed with a productivity of 0.17 g l(-1) h(-1).