Synthetic fibers capable of absorbing aqueous medium are manufactured by simultaneously extruding the fiber material and a second material in a core and sheath arrangement. Turbulence is produced in the zone where these two materials combine which results in undulations in their cross-sections. The second material is then dissolved away with a solvent to leave a fiber with an undulating surface. When the second material is extruded as the core, the fiber is hollow with undulations on the inside while when the second material is extruded as the sheath the solid fiber has an undulating outer surface.