Natural fiber composites were designed and optimized to achieve good mechanical properties and resistance to growth of living organisms. Composite materials were prepared from poly(lactic acid) (PLA) with flax fibers, which had been subjected to interstitial polymerization to replace the water in the cellulose fibers. Prior to the polymerization, the flax fibers were extracted with sodium hydroxide and acetone to remove lignin, pectin, and waxes from the cellulose. Differential scanning calorimetry was used to study the crystallization and melting of the composites compared to pure PLA. The surface wetting of the fibers and morphology of the composites were studied by scanning electron microscopy and optical microscopy. Mechanical properties were studied using dynamic mechanical analysis. The influence of the interstitial polymerization on the dynamic storage modulus was found to be significant. The composites of polymerization treated flax with acetone washed fibers had higher storage moduli than the unwashed fiber composites, which suggested that the adhesion between the flax fibers and the matrix was improved by the treatments. The composites were subjected to moist environmental conditions in order to test for development of mold and fungi, and the acetone washed polymerization treated flax composites were resistant to these growths. (c) 2006 Wiley Periodicals, Inc.