Abstract: A toothbrush (10) has a head portion (14) with a plurality of individual bristles (16) wherein each bristle consists of a smooth stabilizing post (22) which is rigidly attached to the head portion, a smooth tip (18) at a free end of the bristle, and a roughened intermediate portion (20) or portions between the stabilizing post (22) and the smooth tip portion (18). The roughened portions are made in the form of grooves (26) produced by squeezing the bristles between several pairs of die plates such as (40a), (40b) and (42a), (42b), etc. In a squeezed state of the bristles one die plate of the pair is moved in a longitudinal direction. At the same time, the toothbrush is turned back and forth with respect to now-stationary die plates which are maintained in a bristle-squeezing state. As a result, grooves or knurlings which extend in a circumferential direction of the bristles are formed on the side surfaces of the bristles.