Abstract: In the system described for connecting a valve unit for gas water-heaters to a tank of the water-heater, a probe belonging to the valve unit is housed in the tank in a leaktight manner. The system comprises a first element belonging to the valve unit and/or fixed thereto, a second element which projects from the tank and can be housed in a seat of the first element, the second element constituting a support and/or cover for the probe and having an outer surface in which there is a transverse groove, and a third element which is restrained axially on the first element and at least a portion of which extends in the seat in an arrangement such as to engage the groove when the second element is housed in the seat so as to clamp the first and second elements to one another axially, the third element being resiliently deformable between a first position in which the said portion projects into the seat and a second position in which the said portion is outside the seat in order to release the groove.