An electric submersible pump assembly (ESP) (21, 120) is deployed in a production tube (20, 100) in a borehole such that the motor (26, 41, 121) of the ESP is spaced from the inner wall of the production tube, defining a conduit (36, 111) through which the pumped well fluid can flow to cool the motor. The production tube may have an enlarged diameter portion (25, 76, 101) within which the motor is positioned. Alternatively or additionally, the ESP and/or the production tube may be provided with stabilizing spacers (24, 45, 140, 141) which extend between the ESP and the tube to centralize the ESP in the tube and support it against vibrational movement, the spacers defining an annular conduit (36, 111) between the motor casing and the production tube.