Abstract: A submersible pump has a cartridge of pump stages that are compressed between an inlet base and a discharge head as a pump casing surrounding the cartridge is roll-formed to complementally engage shoulders on the base and the head. The inner diameter of the casing presents a slight interference fit with each of the pump stages so that a diffuser on each stage is held in a stationary position during operation of the pump. The casing also provides a plurality of radially inwardly directed forces to maintain the stages in alignment with each other. Complete circumferential contact of the diffuser walls with the interior surface of the casing reduces interstage leaking such that pump efficiency is enhanced.