A catalytic muffler for an internal combustion engine of a portable working tool, e.g. a chain saw or a trimmer, having a housing designed to be directly attached to an exhaust port of an engine. The housing further includes a front chamber and a rear chamber divided from each other by an intermediate baffle. The housing further having an inner housing having a first open end constituting the exhaust gas inlet located in the rear chamber. The inner housing includes a catalyst body through which essentially all exhaust gas flows when in use. A second open end of the inner housing is arranged in one of said front and rear chambers and the exhaust gas outlet of the muffler is arranged in the other chamber such that treated gas in use flows through at least one aperture in the intermediate baffle from one chamber to the other.