The survival of Mycobacterium paratuberculosis during mesophilic (35°C) and thermophilic (53-55°C) anaerobic digestion of slurry was investigated in laboratory batch reactors. At mesophilic temperature, and with initial counts of 3·3 × 10³-2·7 × 10⁴ viable units per gram slurry, the bacteria could be demonstrated on days 7, 14 and 21, but not on day 28. At thermophilic temperature with initial counts of 1·3 × 10⁴ and 2·7 × 10⁴ viable units per gram slurry, the bacteria could not be demonstrated after 24 h in either of the two tests performed, nor after 3 h in the one test where samples were withdrawn at short intervals. The results are discussed in relation to possible practical implications for slurry handling in large-scale biogas plants.