Abstract: This invention relates to an exhaust gas muffler assembly for internal combustion engines which is operable in both a muffled or unmuffled mode. In exterior appearance, the bypass muffler of this invention looks like a conventional muffler having a shell and end caps which enclose the interior components, an inlet for entry of the exhaust gas from the engine, and two outlets for exit of the exhaust gas to atmosphere. A first outlet is preferably positioned opposite to the exhaust gas inlet and connected directly to the inlet by means of a pipe. This pipe is not provided with silencing means. Within the interior of the muffler are silencing means employing a center pipe and an exhaust gas outlet pipe, each of which are provided with silencing means such as apertures in the walls of each pipe. Flow diverting means are removably placed within the first outlet.