A two-stroke internal combustion engine comprising: a piston-cylinder group defining a combustion chamber and a pumping chamber, the piston being mobile in the cylinder between a bottom dead centre, wherein the volume of the combustion chamber is maximum and the volume of the pumping chamber is minimum, and a top dead centre, wherein the volume of the combustion chamber is minimum and the volume of the pumping chamber is maximum, an intake conduit communicating with the pumping chamber, an exhaust conduit communicating with the combustion chamber, at least a pair of transfer conduits able to set the pumping chamber in communication with the combustion chamber, and at least two primary channels fashioned in the piston and individually provided with an inlet which faces onto the exhaust conduit and an outlet which faces on a respective transfer conduit at least when the piston is at the top dead centre; where a lateral surface of the piston comprises a lowering in which the inlets of the primary channels are located.