The researches were undergone on the Danube scomber, species that cumulates the natural microbian contamination from the fresh water of the Danube and from the salty water of the Black Sea. For the eviscerated fish, the aerobic putrefaction takes place prioritarily, placed both on the skin surface and on that which was eviscerated. For the whole fish, the putrefaction is conditioned by the aggression of the intestinal bacterial flora. The value of the peroxide index and the intensity of the Kreiss reaction increase proportionally with the evolution of the rancidity. Between fat hydrolization and oxidation, there is no mutual conditioning, thus, the estimation of the two freshness indices must be made independently one from each other.