Nisin is a widely used naturally occurring antimicrobial effective against many pathogenic and spoilage microorganisms. It has been proposed that reduced efficacy of nisin in foods can be improved by technologies such as encapsulation to protect it from interferences by food matrix components. This study was carried out to evaluate the microbiological quality of fresh trout slices treated with N1 (nisin 0.15 g/kg) and N2 (nisin 0.25 g/kg), NE1(encapsulated nisin 0.15 g/kg), NE2 (encapsulated nisin 0.25 g/kg ) and were then packaged under Modified Atmosphere Packaging (MAP) (45% CO2, 50% N2 ,5% O2) and stored at 4±1 °C for 20 days. The results revealed that nisin in both forms of free and encapsulated was efficient against the proliferation of various categories of spoilage microorganisms including aerobic and psychrotrophic populations and lactic acid bacteria. The shelf life of the treated products was extended by 4–7 days more than that of the control. As a consequence, nisin, in particular encapsulated nisin, might be considered as an effective tool in preventing the quality degradation of the fillet, resulting in an extension of their shelf life.