The tensile properties of Argiope trifasciata (Argiopidae) drag line silk retrieved from mooring threads in the web were characterized. Scanning electron microscope images were used to determine the cross-sectional area of the samples, allowing force-displacement plots to be rescaled as stress-strain curves and to characterize fracture surfaces. Twenty-eight samples were tested to obtain statistically Significant values of the mechanical parameters (elastic modulus, stress and strain at the proportional limit, and tensile strength). The tensile strength of the material was subjected to a Weibull analysis-the first time that this has been attempted with a spider silk. A low value of the Weibull modulus, m = 3.4, was obtained, demonstrating that drag line monofilament does not have a sufficiently reliable tensile strength to function as an engineering material on its own.