Abstract In Norway, fishing is a multibillion-dollar industry with total revenue of over 40 billion NOK yearly for salmon alone. Fish processing plants are prone to contamination of a pathogen bacterium Listeria monocytes. This food-borne pathogen can survive in various environmental conditions allowing it to grow in food processing plants for years that can result in recurring contamination of RTE products, which may cause life-threatening infection to the customers. The bacterium is arduous to control, and companies are seeking strategies to reduce its occurrences. During this study, historical data of ten months period were evaluated to check the presence of L. monocytogenes. The primary purpose of this master thesis is to determine the sources of L. monocytogenes contamination in a Helix tank to prevent the occurrences by understanding the variation of the process and its system using Lean Six Sigma as methodology. Using Lean Six Sigma method has proven feasible to elucidate the sources of L. monocytogenes contamination. The integration of Lean and Six Sigma philosophies with Lean focuses on the identifying the waste of cleaning process and Six Sigma seeks to improve the quality of process outputs by identifying the root cause of L. monocytogenes contamination. Thus, combining these two methods compensates each other’s weaknesses and boosts its strengths. Lean Six Sigma’s DMAIC approach and the bacteria detection, the NordVal no:022 method and combination with conventional plating are used. Results show that the sources of L. monocytogenes contamination in Helix 1 tank is caused by the cross-contamination from the boat having 32% correlation and seasonal variation with 23% correlation, having higher frequency on spring and autumn with 38% and 41% respectively while in winter and summer has 18% and 3%. The DNA based MLVA method was also applied to subtype and compare L. monocytogenes obtain from the factory. One L. monocytogenes subtype that dominant (green coded subtype) and is found in the factory and boats having 56% (18 of 32) and 34% (17 of 50) occurrence respectively. Among the L. monocytogenes occurrences in Helix 1 tank, 41% (13 of 32) of it has the same MLVA profile from the boat and the other 59% (19 of 32) are district subtypes. Moreover, four types were found present to match MLVA profiles from the end products; HOG, Feeder, and Filet with 32% (11 of 32), 19% (19 of 32) and 9% (3 of 32) respectively. The result of this study showed that Helix 1 is the primary controllable source of the L. monocytogenes contamination in the plant, thus having a high risk to contaminate the end products. However, based on the result of this master thesis it cannot be concluded that only boats, seasons of the year are the main contributors of the contamination, there could be other factors that influenced the contamination in Helix 1 but is not included in this study.