An amphiphilic poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymer (BCP) was blended with a bisphenol A-based epoxy resin formulation and self-assembled into a wormlike micelle structure. With an incorporation of 5 wt % of the BCP material, the fracture toughness was improved by > 100% over the neat epoxy. The morphology and mechanical properties of this BCP-modified epoxy were investigated using transmission electron microscopy, dynamic mechanical analysis, tensile tests, and fracture toughness measurements. Toughening mechanisms from the wormlike micelle-modified material were investigated using the double-notch four-point-bending technique, and the results are compared with data obtained from the same epoxy thermoset formulation containing a BCP that self-assembled into spherical micelles. Elongated cylindrical micelles produce improved toughness, which is interpreted on the basis of a combination of mechanisms including crack tip blunting, cavitation, particle debonding, limited shear yielding, and crack bridging. The implications of the present study for polymer toughening in general are also discussed.