Multiple filament acetate tow (`M' _ Tow) is composed of a bundle of cellulose acetate filaments having at least two different filament denier sizes. The flow of cigarette smoke in `M' _ Tow filters was analyzed. The blocks of fine filament denier and the blocks of large filament denier are arranged longitudinally and randomly distributed over the cross-section of the filter. These blocks influence the smoke-flow pattern. During puffing, a pressure differential is created across the filter in the axial direction and, due to the difference of filament deniers, also in the transverse, or nonaxial, direction. Consequently, smoke flows both transversely and axially and it passes laterally through the blocks of fine filament denier and flows into the blocks of large filament denier. This trend serves to increase the effective surface area of the filter, and is attributable to higher filtration efficiencies than the conventional filters at the same pressure drop.