Increased demand of fish in combination with overexploitation of the fish stocks of the oceans has led to an increased production of fish through aquaculture. Today, fishmeal is the main protein source in fish feed for most aquaculture species. However, fishmeal is soon expected to fall short of demand as its production is associated with environmental problems. This shortage must therefore be met by sustainable alternative protein sources. Protein-rich microorganisms (i.e. Single cell protein) is an interesting option as a fishmeal substitute in fish feed which, in addition, can be produced as an important co-product in wood-based biorefineries. In the current study, four different microorganisms were cultivated on five different residual streams from Swedish wood-based biorefineries. Screening experiments were carried out in shake flasks, optimization experiments in benchtop bioreactors, and scale-up experiments were performed in a 50-litre pilot bioreactor. In addition, a demo-scale experiment was carried out in the Swedish Biorefinery Demo Plant. Microbial biomass from the scale-up experiments was collected and used for production of different fish feed formulations which, in turn, were used in feeding trials of the freshwater fish Tilapia. Fishes fed with feed, in which part of the fishmeal had been substituted with Single cell protein, showed similar or better growth than fishes fed with a fishmeal-based control feed.