Abstract: A tube coupling having an arcuate grip surface which engages with and secures tubing with minimum scarring and distortion of the tubing. The grip surface is rotatable to successively present arcs of smaller radius to the tube whereby the gripping engagement tends to occur over a long line of contact. One embodiment employs a right-circular cylindrical cavity only slightly larger than the tube to be coupled for reception of the tube and a rotatable locking element mounted normal to the cavity and intersecting with the cavity to present a gripping surface to the cavity which upon rotation in a biased direction presents a groove defining in the cross-section of the cavity a succession of arcs of progressively smaller radius to the cavity and a tube therein until an arc conforms to and grippingly engages the tube as it bears against an opposing cavity wall. Various seal structures of elastomeric material are accommodated within the coupling body.