The double volute casing of a centrifugal pump has a main portion which has first and second volutes extending along arcs of not more than 180 degrees. The outer diameter of the main portion equals or approximates the outer diameter of the end of the first volute, and such end is adjacent to the start of the second volute whose end communicates with an outlet nozzle of the main portion. The latter has two channels which flank the second volute and extend between the end of the first volute and the outlet nozzle. The inner sides of the channels are open and face the axial passage of the main portion. Such inner sides are sealed from the axial passage by tubular inserts which are installed in and are separably fastened to the main portion of the casing. The passage receives a single-suction or a double-suction impeller or a stationary guide wheel which is located downstream of the impeller. The main portion of the casing is a one-piece casting.