Natural rubber is reinforced with untreated coir fiber chopped to different lengths, viz., 6, 10, and 14 mm. Mixes were also prepared using 10 mm-long coir fibers treated with 5% sodium hydroxide solution for different time intervals, viz., 4, 24, 48, and 72 h. These composites were vulcanized at 150 degrees C. The vulcanization parameters, processability characteristics, and stress-strain properties of these composites were analyzed. The rubber-coir interface bonding was improved by the addition of a resorcinol-hexamethylenetetramine dry-bonding system. The reinforcing property of the alkali-treated fiber was compared with that of the untreated one. The extent of fiber orientation in the composite was determined from green strength measurements. From anisotropic swelling studies, the extent of fiber alignment and the strength of fiber-rubber interface adhesion were analyzed. Scanning electron microscopic studies were carried out to investigate the fiber surface morphology, fiber pullout, and fiber-rubber interface.