Abstract: A coupling between first and second tubular members comprising a flexible retaining ring fitted over the first tubular member below an annular enlargement thereof and entering into screw-threaded engagement with internal screw-threading on the second tubular member or a connector element attached thereto. The retaining ring is a split-ring member whose longitudinally extending slit enables it to be passed lengthwise over the first tubular member and is so dimensioned that on re-assuming its original form, the opposed margins of the slit abut and the retaining ring is held against the first tubular member against removal therefrom by the enlargement, and the second tubular member or connecting member is coupled to the first tubular member. A sealing member is positioned at the coupling to form a fluid-tight seal at the junction of the tubular members.