Fish has for a long time been a very important source of protein for human kind and with the world population at an all-time high, 7.5 billion and rapidly growing, the demand for fish as a food source is also at an all-time high and rapidly increasing. This has in turn led to overexploitation of many of the fish stocks of the world ocean’s and in many cases to depletion of fish stocks. The demand for sustainable food sources and sustainable usage of the world ocean’s fish stocks is therefore a subject with great deal of interest today. Much of the fish caught today are used for production of fish meal for usage as fish food at fish farms, which also increases the depletion of fish stocks around the globe. One way of dealing with this problem is to replace the fish meal as protein source in fish feed with protein from agricultural crops which in many cases are done today by usage of soy bean protein. This however poses another problem as the agricultural crops take up vast amount of land, in many cases obtained by diminishing the rainforests in the area. Another usage for the soybean would be as a direct human food source. Agricultural products are also dependent on environmental conditions to ensure reasonable production. The problems related to production of fish meal and soy has sparked the idea of using microorganisms for production of Single cell protein for usage as protein source in fish feed. Single cell protein can be produced in closed fermentation vessels and can be produced at a controlled rate and under controlled manners, while taking up negligible land space. During this thesis, the production of single cell protein from spent sulfite liquor using the filamentous fungi Paecilomyces variotii was examined. The aim of the project was to examine the effect of cultivation parameters (i.e., pH, temperature and nutrients) on the production of biomass as well as the protein content of the biomass. The correlation of the biomass growth and protein content have also been examined. The project was carried out by performing several experiment cultivations using spent sulfite liquor provided by Domsjö Fabriker in Örnsköldsvik. This process enables the utilization of a residual stream from the pulp industry which gives this process a huge environmental upside compared to similar processes as for example the commercial production of Quorn (a Single cell based food product) which utilizes pure glucose. The results showed that the protein content will steadily decrease as the biomass production increases hence it is desirable to keep the cultivation time at a minimum while maximizing biomass production during that time frame. It also points towards that the highest protein content is present in the young cell mass. The key conclusion from this thesis is however that it is possible to lower the pH of the cultivation from pH 6 down to pH 4.5 while still maintaining the biomass production and increasing the protein content. The highest obtained protein content was 62.7% at pH 4.5. The high protein content might be due to a slightly longer lag phase in the beginning of the cultivation which yields a higher number of younger cells in the final broth thus increasing protein content. Running the process at a lower pH is a huge advantage for industrial implementation as this on large scale means significant lower amounts of chemicals needed for pH adjusting of the spent sulfite liquor which renders the process much more economical. This is because pH adjustment today is one of the most costly process steps in the production of bioethanol from spent sulfite liquor.