In recent years reports have been published on non-sterility problems in UHT milk caused by the survival of very heat-resistant spores, which have been identified as belonging to the species Bacillus sporothermodurans. In order to solve the problems in dairy practice more information is needed on the thermal death kinetics of these spores. The heat resistance of spores of three Bacillus sporothermodurans strains isolated from non-sterile UHT milk was determined in the temperature range of 110–145°C and was compared with the heat resistance of Bacillus stearothermophilus spores. For the low temperatures (110–125°C) the heating was carried out in tubes. For the higher temperatures (130–145°C) a direct UHT sterilizer was used. D140 values of 3.4–7.9 s (B. stearothermophilus D140=0.9 s) indicate an exceptionally high heat resistance of spores of B. sporothermodurans under UHT conditions. Thermal death time (TDT) curves show different slopes for B. sporothermodurans and B. stearothermophilus, with z=13.1–14.2°C and 9.1°C, respectively. To our knowledge there are no observations of other spores in the literature for which such high D140 values and z values have been demonstrated.