Device to be applied to a ring spinning machine for the production of patterned yarns wherein the fibres coming from a rove which constitutes the base rove (S) are ' covered by a rove (T) made of fibres of different nature and/or colour; In each position of the spinning machine the two roves (S and T) advance parallel and separate on the same plane detected by the middle cylinder (M) and on output cylinder (F) with related pressure roller (M' and F' ) and by supporting belts dragged by said middle pressure roller (M' ); the middle pressure roller (M') and the related supporting belt (R') are divided into two parts, one part of the roller and the related belt (HB) acting on the base roves (S), while the other part of the roller and the related belt (HC) act on the roves (T) of different nature and/or colour; the two parts of the pressure roller (M') and of the related belts (HB, HC) being on the same axis, but being provided with means which independently rotate them, so that the two roves can have different drawings.