Aims: The aim of this study was to identify and characterize heat stable proteinases of psychrotrophic proteolytic bacteria isolated from raw milk. Methods and Results: A strain of Klebsiella oxytoca producing a high proteolytic activity when cultured on milk was isolated. Maximum proteolytic activity was observed at the stationary phase during growth on milk or caseinpeptone broth. The bacterium demonstrated the capability to grow at 7°C, classified as psychrotrophic. The crude enzyme showed optimum activity at 37°C, and pH 5AE0 and 7AE0. The proteinase was very resistant to heat, maintaining 74% of initial activity after incubation at 142°C. Conclusions: A heat stable protease of a psychrotrophic strain of K. oxytoca was identified and partially characterized. Significance and Impact of the Study: Thermal stable proteases may constitute a serious problem to ultra-high temperature (UHT) processed milk, leading to undesirable physical and sensory alterations.