Bacterial-cellulose nanofibers obtained from Acetobacter xylinum can reinforce polymer resins while maintaining the transparency of the original resin, even at fiber contents as high as 70 wt.-%, because the nanofibers do not appreciably scatter visible light. The flexible plastic composites (see Figure) reinforced with this renewable resource have thermal expansion coefficients of 6 x 10(-6) degreesC(-1), Young's moduli of 20 GPa, and tensile strengths reaching 325 MPa.