Abstract: A multi-stage pump embodying an inner housing of modular construction and embodying a plurality of axially aligned casing rings having impellers rotatably mounted therein, the casing rings being removably mounted relative to each other and, together with the impellers disposed therein, each comprising a respective stage for the pump, with an outer housing mounted around the inner housing, in spaced, surrounding relation thereto to thereby afford an annular space between the inner and outer housings, with end sections being releasably secured to opposite ends of the outer housing in position to clamp the modules of the inner housing together, and with a passageway through the pump being afforded by the casing rings, the impellers, a radial opening through one end of the inner housing, the space between the inner and outer housings, a radial opening through the outer housing and a connector in communication with the last mentioned opening.