The efficacy of a multistage ejector pump is considerably improved by flow guiding profiles (1, 2, 3; 4) inserted into the flow channel (17, 17', 17'). The flow guiding profiles consist of one or several adjacent and/or successive profiles which on the one hand reduce the turbulence resulting from the mixture of the substance to be conveyed with the pump fluid and consequent damages to subsequent pump stages, and on the other hand help to reduce the residual energy of the pump fluid. The flow guiding profiles are designed in such a way that they can be used both in so-called flat channel pumps and in radial ejector pumps. The flow guiding profiles can further be assembled with corresponding segments of the ejector pump housing and profiles into ejector pump modules capable of being produced in a single piece and of being assembled with other similar modules into pumps having the desired capacity.