The invention concerns a device (1) for forming a flow (4) of tobacco fibres whereof each cross-section contains the same amount of tobacco fibres uniformly distributed, starting from a mass (2) of tobacco fibres contained in a reservoir (3). Said device (1) comprising: an apparatus (6) for drawing, from the reservoir (3) successive portions (7) of tobacco and for transferring said portions (7) to a so-called overflow zone (8) above at least a ridge (8A) which extends more or less transversely to the flow (4) to be formed; in the so-called overflow zone (8), an apparatus (9) for receiving said successive tobacco portions (7) so as to continuously assemble said portions to form a layer (4A) and transfer the tobacco forming said layer (4A) to another station (10).