Method for preparing a biomass of Thraustochytrids, a substantially majority quantity of which is not degraded, comprising proteins and fatty acids, characterised in that it comprises, by weight relative to the weight of dry matter, at least 40% proteins and at least 20% fat, the fat having a content of 25 to 60% polyunsaturated fatty acids selected from arachidonic acid, docosahexaenoic acid and eicosapentaenoic acid, without added proteins or fatty acids, characterised in that it comprises: a. a first step of culturing Thraustochytrids carried out in so-called "batch" mode, in so-called "fed batch" mode or in continuous mode under heterotrophic or mixotrophic conditions in a defined culture medium comprising a carbon source, a nitrogen source, a phosphorus source and salts, at a temperature between approximately 24°C and 35°C and under conditions for promoting the production of proteins at a content of at least 40% protein by weight relative to the weight of dry matter, step a) been divided into two sub-steps, a first growth sub-step a1) in the appropriate culture medium until obtaining carbon source contents in the medium less than 20 g/L, followed by a second, production sub-step a2) in which one or more solutions for enrichment in carbon source, nitrogen source and phosphorus source are simultaneously or successively added to the culture medium, the content of carbon source is maintained between 0 and 50 g/L, the content of nitrogen source is maintained between 0.5 and 5 g/L, and the content of phosphorus source is maintained between 0.5 and 5 g/L; b. a second step of culturing under heterotrophic or mixotrophic conditions at a temperature between approximately 18°C and 25°C and less than that of step a), under conditions for promoting the accumulation of fat up to at least 20% by weight relative to the weight of dry matter, and until obtaining a culture density of at least 40 g/L dry matter; c. a third step of recovering the biomass obtained in the second step, by separating said biomass from the culture medium (the harvest); and, if necessary d. a fourth step of drying the biomass recovered in the third step. a. a first step of culturing Thraustochytrids carried out in so-called "batch" mode, in so-called "fed batch" mode or in continuous mode under heterotrophic or mixotrophic conditions in a defined culture medium comprising a carbon source, a nitrogen source, a phosphorus source and salts, at a temperature between approximately 24°C and 35°C and under conditions for promoting the production of proteins at a content of at least 40% protein by weight relative to the weight of dry matter, step a) been divided into two sub-steps, a first growth sub-step a1) in the appropriate culture medium until obtaining carbon source contents in the medium less than 20 g/L, followed by a second, production sub-step a2) in which one or more solutions for enrichment in carbon source, nitrogen source and phosphorus source are simultaneously or successively added to the culture medium, the content of carbon source is maintained between 0 and 50 g/L, the content of nitrogen source is maintained between 0.5 and 5 g/L, and the content of phosphorus source is maintained between 0.5 and 5 g/L; b. a second step of culturing under heterotrophic or mixotrophic conditions at a temperature between approximately 18°C and 25°C and less than that of step a), under conditions for promoting the accumulation of fat up to at least 20% by weight relative to the weight of dry matter, and until obtaining a culture density of at least 40 g/L dry matter; c. a third step of recovering the biomass obtained in the second step, by separating said biomass from the culture medium (the harvest); and, if necessary d. a fourth step of drying the biomass recovered in the third step.