The reductions of pathogenic and indicator bacteria in animal slurry subjected to mesophilic (35°C) and thermophilic (53°C) anaerobic digestion have been measured in small-scale, as well as full-scale, reactors. For all vegetative bacteria a time-dependent inactivation was observed. At small-scale digestion at 35°C average T₉₀ values were 2·4 days for Salmonella typhimurium, 2·0 days for S. dublin and Streptococcus faecalis, 1·8 days for Escherichia coli and Erysipelothrix rhusiopathiae, 0·9 days for Staphylococcus aureus and 7·1, 3·2 and 3·1 days for group D-streptococci, faecal coliforms and total coliforms of the indigenous flora. At 53°C T₉₀ values of 1·2 h were determined for E. rhusiopathiae, 1·0 h for Str. faecalis, 0·7 h for S. typhimurium, 0·6 h for S. dublin, 0·5 h for Staph. aureus and 0·4 h for E. coli. Spores of Clostridium perfringens type C and Bacillus cereus were not inactivated at 35°C or 53°C. The value of T₉₀ was significantly correlated to species of bacteria and temperature during digestion, but in general it was not influenced by the type of slurry (cattle or pig), the reactor process (batch or continuous), the amount of gas produced during fermentation, the slurry dry-matter contents, the concentration of NNH₃ and pH in the slurry. T₉₀ values for S. typhimurium and E. coli at full-scale digestion were fully comparable to the values determined by small-scale experiments.