Abstract: The collar is embodied in the form of a single annular slotted piece (1) comprising a base body (2) and two ends (3, 4) provided with corresponding opposite toothings (7, 9) which are engageable into each other and each of which is provided with a row of teeth inclined in a direction opposite to the collar closing direction (F). An intermediate part of the base body (2) is corrugated in such a way that a hinge (10) is formed. A plurality of flexible tabs (11), which are inclined towards the hinge (10) and radially inwardly projected, is circumferentially distributed through a surface radially internal with respect to the base body (2). Said invention is suitable for connecting flexible or semi-flexible conduits single or double skinned, in particular for ventilation system air-conveying conduits.