An experimental investigation on the properties of fresh and hardened mortars reinforced with subdenier monofilament polypropylene fiber (MMPPF) is reported. Fiber effects on properties of mortars were assessed. Properties studied were flow, unit weight, setting time, bleeding ratio, compressive strength, impact resistance, water infiltration, and flexural load-deflection. The results revealed that the addition of MMPPF had a positive effect on workability of fresh mortars, impact resistance, water impermeability, and flexural toughness, but had no positive effect on compressive strength. (C) 2004 Wiley Periodicals, Inc.