Destabilizations of casein micelles can be observed after modifications of physico-chemical conditions or their proteolysis with as consequence gelation or sedimentation. In this context, the objective of this work were to 1.appreciate the variability of this destabilization of UHT milk during their storage as a function of the strains of Pseudomonas and 2.understand the physico-chemical modifications of casein micelles induced by 9 strains of Pseudomonas fluorescens and also by AprX, an extracellular protease produced by one strain of Pseudomonas fluorescens F. After inoculation of raw milk by these strains or after addition of different concentrations of purified AprX protease in raw milk and UHT treatment, milk destabilization was determined at macroscopic, colloidal and molecular levels. For experiments testing the strain variability, 5 on the 9 tested strains were highly destabilizing. For experiment with purified AprX enzyme, destabilizations were also observed. In all these cases, instabilities were visual (presence of sediment) and increasing as a function of time (after several days or weeks depending on the strains and the concentrations of added enzyme). The analyses of the destabilized UHT milks revealed the presence of aggregates. The zeta potentials and hydrations of casein micelles decreased. At molecular level, we determined a significant proteolysis. The determination by liquid chromatography coupled to mass spectrometry of one part of the released peptides indicated that the α s1-, α s2-, β - and κ-caseins were hydrolyzed with a quantitative preference for β -casein. The nature of the different peptides released was similar for all destabilized UHT milks. The decrease in the stability of casein micelles will be discussed in relation with the modifications of structure and the observed proteolysis. Potential application of this research in term of detection of unstable UHT milks will be proposed knowing that this enzyme is heat-resistant and its implication in the destabilisation of UHT milk during its storage is often evoked.