A hybrid drive system for a utility vehicle (1) which operates with an internal combustion engine (12) and an electrical generator (80), motor (86) and storage means (84). Auxiliary hydraulic functions (93,94) may be powered by the electric motor (86). An output of the engine (12) is mechanically coupled to a multi-ratio transmission (10) which has a hydrostatic driveline (24) and an output which is mechanically coupled to propulsion means. With the engine switched off, the electric motor powers a hydraulic pump (91) which generates pressurized fluid to operate auxiliary functions. In addition, the pressurized fluid may be exploited in a secondary propulsion mode to drive a hydraulic motor (92) in the hydrostatic transmission to propel the vehicle in a low emission, low noise output mode.