Abstract: A method for manufacturing a suspension of cellulose nanofibrils from an aqueous suspension of cellulose fibres is disclosed including the following steps: chemical treatment, in a reactor, of the aqueous suspension of cellulose fibres by at least one functionalising agent, the chemical treatment being configured to functionalise the cellulose fibres at a modification rate that is higher, indeed strictly higher, than 1.0 mmol/g, and mechanical treatment, in a twin-screw extruder, of the aqueous suspension of functionalised cellulose fibres, each screw of the extruder comprising at least two fibrillation segments. In accordance with at least one embodiment, this method makes it possible to produce a suspension of cellulose nanofibrils of good quality, with a high dry matter content, and while consuming less energy than the methods of the prior art.