The properties of some polyamide fibers - polycaproamide (PCA), poly-p-amidobenzimidazole (PABI), poly-p-phenyleneterephthalamide (PPTA), poly-m-phenyleneisophthalamide (PPIA), and polyimide (PI) - treated by different micro-organisms that are abundant in nature (the bacteria Bacillus and Pseudomonas, and the fungi Aspergillus) were studied. The interfacial bond strength between these fibers (untreated and exposed to micro-organisms) and thermoplastic polymers such as polyethylene (PE), polycarbonate (PC), and polysulfone (PSF) was investigated by the single-filament composite and pull-out techniques. If the treatment time is not very long (1-3 weeks), the tensile strength of the fibers does not deteriorate. However, the topography of the surface undergoes considerable changes in some cases. The result of the treatment depends substantially on the chemical nature of the nutrient as well as on the micro-organism species. Two main avenues for fiber modification are possible: (1) decomposition of macromolecules in the surface layer, resulting in surface growth; and (2) adsorption of the metabolic products, which heal surface flaws and alter the chemical nature of the surface. The increase in the interfacial shear strength (IFSS) for the modified fiber-thermoplastic matrix systems was from 15% for the PCA/PE composite to 80% for the PABI/PSF and PABI/PC pairs. This increase was especially marked when a sublayer of a substance specially added to the nutrient, having better compatibility with the matrix, was formed by micro-organisms on the fiber surface.