A method for producing a measuring tube for a flowmeter, in particular for a magneto-inductive flowmeter or an ultrasonic flowmeter or a vibronic flowmeter, wherein the measuring tube has an, in particular at least sectionally hollow-cylindrical, support tube with a wall made of an, in particular non-ferromagnetic, metal and with a lumen surrounded thereby, in particular having at least sectionally circularly cylindrical or circular cross sections, a liner lining the inside of the support tube and at least one sensor element attached to the support tube to detect at least one measurement variable of a fluid carried in the measuring tube, which method comprises: - providing the support tube; - forming the liner in the lumen of the support tube; - and applying the at least one sensor element to the support tube; wherein the forming of the liner comprises: - providing a liner tube (3*) with a wall made of a thermoplastic, in particular a polyethylene or a polyvinyl chloride, and a lumen surrounded thereby, -- wherein the liner tube (3*) has an external diameter which is smaller than an internal diameter (caliber) of the support tube -- and wherein, in a first section (3a*), the wall of the liner tube (3*) has at least one fold extending, in particular, along an imaginary longitudinal axis, such --- that both the lumen and a lateral surface of the liner tube each have a form deviating from a circular cylinder, in particular a channel-shaped form, and a non-circular cross section, in particular a c-shaped or u-shaped cross section --- and that the lateral surface of the liner tube (3*) is concavely curved in the first section (3a*); - positioning the liner tube (3*) in the lumen of the support tube; - heating the liner tube (3*), in particular by means of steam introduced into the lumen of the liner tube (3*), to put the plastic of the wall of the liner tube (3*) into a viscoelastic state; - deforming the wall of the liner tube (3*) such -- that the wall of the liner tube (3*) unfolds and is pressed onto the wall of the support tube -- and that the lateral surface of the liner tube (3*) is convexly curved in the first section (3a*); - and cooling the liner tube (3*) or allowing the liner tube (3*) to cool in order to put the plastic of the wall of the liner tube (3*) into an elastic state such that a shape and/or force-based locking affixing the liner tube (3*) permanently in the support tube is formed between liner tube (3*) and support tube. - providing the support tube; - forming the liner in the lumen of the support tube; - and applying the at least one sensor element to the support tube; - providing a liner tube (3*) with a wall made of a thermoplastic, in particular a polyethylene or a polyvinyl chloride, and a lumen surrounded thereby, -- wherein the liner tube (3*) has an external diameter which is smaller than an internal diameter (caliber) of the support tube -- and wherein, in a first section (3a*), the wall of the liner tube (3*) has at least one fold extending, in particular, along an imaginary longitudinal axis, such --- that both the lumen and a lateral surface of the liner tube each have a form deviating from a circular cylinder, in particular a channel-shaped form, and a non-circular cross section, in particular a c-shaped or u-shaped cross section --- and that the lateral surface of the liner tube (3*) is concavely curved in the first section (3a*); -- wherein the liner tube (3*) has an external diameter which is smaller than an internal diameter (caliber) of the support tube -- and wherein, in a first section (3a*), the wall of the liner tube (3*) has at least one fold extending, in particular, along an imaginary longitudinal axis, such --- that both the lumen and a lateral surface of the liner tube each have a form deviating from a circular cylinder, in particular a channel-shaped form, and a non-circular cross section, in particular a c-shaped or u-shaped cross section --- and that the lateral surface of the liner tube (3*) is concavely curved in the first section (3a*); --- that both the lumen and a lateral surface of the liner tube each have a form deviating from a circular cylinder, in particular a channel-shaped form, and a non-circular cross section, in particular a c-shaped or u-shaped cross section --- and that the lateral surface of the liner tube (3*) is concavely curved in the first section (3a*); - positioning the liner tube (3*) in the lumen of the support tube; - heating the liner tube (3*), in particular by means of steam introduced into the lumen of the liner tube (3*), to put the plastic of the wall of the liner tube (3*) into a viscoelastic state; - deforming the wall of the liner tube (3*) such -- that the wall of the liner tube (3*) unfolds and is pressed onto the wall of the support tube -- and that the lateral surface of the liner tube (3*) is convexly curved in the first section (3a*); -- that the wall of the liner tube (3*) unfolds and is pressed onto the wall of the support tube -- and that the lateral surface of the liner tube (3*) is convexly curved in the first section (3a*); - and cooling the liner tube (3*) or allowing the liner tube (3*) to cool in order to put the plastic of the wall of the liner tube (3*) into an elastic state such that a shape and/or force-based locking affixing the liner tube (3*) permanently in the support tube is formed between liner tube (3*) and support tube.