Abstract: A casing 10 of a pump has first and second casing elements 20, 30. Each of the casing elements has a mating face and these mating faces are sealingly joined. An interior of the first casing element defines a portion of a chamber 50 in which the impeller is mounted. A first interior portion of the second casing element defines the remaining portion of the impeller chamber, and a second interior portion defines a diffusion chamber 60 that is in fluid communication with the impeller chamber. Fluid entering the casing at a fluid inlet 34 passes into the impeller chamber, where velocity is imparted to the fluid. The velocity-enhanced fluid passes axially into the diffusion chamber, which has an expanding cross-sectional area along its length, to convert the velocity into pressure head, which is achieved increasing by at least one of: the radially-inward width and the axial height.