A walk-behind self-propelled working machine has a machine body, a working unit mounted on the machine body to perform a working operation, an engine mounted on the machine body to drive the working unit, and a traveling unit mounted on the machine body and drivable to cause the machine body to travel. A control unit controls operations of the working unit and the traveling unit. An operating section is operatable by a human operator to provide signals to the control unit to control operations of the working unit and the traveling unit. A mode selector switch is mounted on the operating section for operation by the human operator to select one working operation mode from among plural alternative working operation modes that are set in advance in the control unit on the basis of a combination of a travel speed of the machine body, a revolving speed of the engine, and an opening of a throttle valve associated with the engine.
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