Abstract: A coupling for interconnecting two fluid flow lines includes a first coupling member (1) having a male end (4) and a second coupling member (2) having a female end (17) within which a rotatable collar (3) is held captive. The male end (4) includes four equally circumferentially spaced lobes (16) and the collar (3) has a cylindrical portion (3) that is shaped at its inner end to provide a square cross-section (35) to allow passage of the lobes (16) therethrough. The end rim (33) of collar (3) is recessed (37, 38) whereby upon insertion of male end (4) through collar (3) and rotation of the collar, the lobes (16) can seat within the recesses (37, 38), thereby releasably interconnecting the first and second coupling members (1, 2).