Single cell protein (SCP) is a bulk of dried cells which can also termed as bioprotein, microbial protein or biomass. SCP is produced by the microorganisms such as algae, yeast, fungi and bacteria, however, fungi and bacteria are the major producers of this protein. High production of proteins from these sources was mainly due to their fast growth rate and relatively higher protein level in their chemical structure. Some algal species were also used for this purpose which specifically cultivated in the aquatic medium. In addition to high content of protein, SCP also contains carbohydrates, nucleic acids, fats, minerals and vitamins. Additionally, SCP has a high level of essential amino acids such as lysine, methionine, and threonine. This source of protein (SCP) has been proved a good replacement of other expensive protein sources like fish and soybean meals. Therefore, conclusion can be made that SCP can easily replace traditional (plant and animal) protein sources in human, animal as well as fish diets without any detrimental effect. Finally, in this review, we focus on new feeding trials on SCP in some aquaculture species, such as Atlantic salmon, white leg shrimp and rainbow trout.