Tri- or tetra-locular paint brush bristles

In a monofilamentary paintbrush bristle of thermoplastic polymeric material having a diameter of about from 4 to 20 mils (0.10 to 0.51 mm), the improvement wherein the filament is tri- or tetra-locular and comprises three or four enclosed, longitudinal, noncircular voids, the voids being separated by an interior web, the cross-sectional area of the filament having a void content of about from 20 to 50%, and wherein the interior web comprises about from 10 to 25% of the total cross-sectional area of the filament.