Abstract: A water turbine (1) comprises a turbine chamber (3), an inlet port (4) and an outlet port (6) opening into the chamber (3), and two or more rotors (11, 12) arranged within the chamber (3) with their axes of rotation parallel so as to be rotatable in the water flow passing between the inlet port (4) and the outlet port (6). Each of the rotors (11, 12) has two or more substantially equi-angularly distributed lobes (17, 18), and the arrangement is such that the rotors (11, 12) rotate in opposite directions with their lobes in mesh with one another under the action of water impinging on the lobes of the rotors. Such a turbine can be constructed to operate efficiently with a low head of water, and is additionally of simple construction and therefore inexpensive to produce.