Twin-screw extrusion of cornstarch produces a strong, yet readily biodegradable plastic material. Because of the brittleness of these cornstarch plastics, ASTM standard methods did not apply, and an alternative sample grip was developed. A suitable method for the tensile testing of extruded cornstarch plastics of differing feed composition (amylose content, cross-linking, plasticizer) and processing conditions (temperature) was developed, and quantitative values of tensile strength, strain, elastic modulus, and energy were obtained. The mean tensile strength of these materials ranged from 667 to 4148 psi, which is in the range of values for low-density polyethylene and high-density polyethylene. The high amylose content cornstarch plastics extruded at high temperatures demonstrated greatest strength. Wet/dry studies of the pure cornstarch materials showed them to degrade readily with extended exposure to water, but immersion in water for 1 h in room temperature (17 degrees C) water did not, as measured by standard tensile tests, adversely affect their strength, but made them more ductile, less stiff and tougher.