The present invention relates to a method for the production of fat with a principal application as transportation biofuel or a component or raw material therefor. According to the method, cell masses, cell suspensions and/or liquid phases formed in the production of single cell oil, and/or biomass-containing side streams or microorganism cell masses for another purpose and/or originating from other sources, are contacted with a fat-production capable microorganism and the organism is allowed to produce fat. The resulting fat is recovered or the microorganism mass is passed to a single-cell oil production process. By means of the invention, the organic matter present in the cell mass and side streams of single-cell oil can be re-utilized for the production of the single-cell oil, thereby improving a total fat yield, as well as reducing an organic load of the side streams.
