A submersible positive-displacement pump comprises a pump housing (10A/10B) having an inlet (11) at its lower portion and an outlet (17-19) at its upper portion, a variable-volume pump chamber (21) which is provided at the lower portion (10A) of the pump housing and communicates with the inlet through an annular passage (29) having a one-way inlet valve (23) and communicating with the outlet through an opening (20) controlled by a one-way outlet valve (30). A displacement member (27) below the pump chamber (21) is connected with a lifting member (28) which extends upwardly from the pump housing and is connected to a driving device. By means of the lifting member the displacement member can be reciprocated vertically for reducing and increasing the volume of the pump chamber. The pump chamber (21) is defined partly by a circumferentially extending pump housing wall (22) and partly by a sealing coolar (23) which bridges the annular passage (29) and one circumferential end of which is movable in the pump chamber towards and away from the pump housing wall (22) in order to open and close the passage under the influence of a pressure differential between the pump chamber (21) and the inlet (11).