Successive increments of a thermoplastic tube are extruded in vertical direction from an orifice, and a first partible mold is closed about the increment immediately downstream of the orifice whereupon the increment is expanded to the shape of a first semi-finished container, while simultaneously completing the expansion of the neck region of an already filled second semi-finished container accommodated in a second partible mold downstream of the first mold and communicating with the interior of the first container, is completed. The containers are then sealed and separated from one another and the first container in the first mold is filled. The second container is discharged from the second mold and the relative positions of the molds are reversed in preparation for repetition of the preceding steps.