Hydroelectric power plant containing a flow tube for the water, an inlet tube leading to the flow tube and a discharge tube leading from the flow tube. In the flow tube a turbine is arranged to be driven by the flowing water and which via a drive shaft drives an electric generator. Accentuated sub-divisioning as between mechanical unit and portions of an installation nature is provided. The turbine and generator are located in the direct vicinity of each other and together with the drive shaft form a unit which in its entirety is situate in the flow tube and arranged to be traversed by flowing water. The unit is so arranged that the turbine can be in contact with the water flow while the generator has a watertight enclosure into which the drive shaft extends through a watertight bushing. Furthermore an electric cable for transmitting the electricity produced is connected. The installation components, the said tubes, are made from prefabricated concrete components. The flow tube is essentially vertical and exhibits a support for the unit and, at its upper end, an aperture through which the unit can be lowered.