An assembly (1) of a tapping keg (2) with neck (5) and a connecting device (15) for placement on said neck (5). In the neck is at least one valve (20, 22). The neck is provided with an outwardly reaching flange (8) with a top surface (14) and an inclined bottom surface (9). The bottom surface (9) is inclined with respect to the top surface (14). The connecting device has a substantially horse shoe-shaped connecting element (16), provided with a slot (18) by way of which the connecting element (16) is slidable over the flange (8). The side of the slot facing the bottom surface (46) during use comprises a lower surface (19) which is inclined at an angle which is at least virtually equal to the angle of inclination of the inclined bottom surface (9) and, in coupled condition, abuts at least against a planar central part (9A) of the inclined bottom surface (9).