Abstract: A fitting for high pressure gas lines and the like includes a fitting member having an internal bore therethrough. The bore has a portion thereof with an acircular cross section in a plane normal to the axis of the bore. The fitting is connected to the end of the conduit tube by sliding the tube into the bore and forcibly inserting an expander member into the inside of the tube. The expander member diametrically enlarges the tube, pushing its wall outwardly against the acircular portion of the bore. The expander member has a central passage therethrough which aligns with the bore and thus permits gas to flow from the tube through the expander member and into a second fitting which is connected to the fitting member.