The fracture toughness of cold-worked and stress-relieved Zircaloy-4 sheet subject to through-thickness crack growth within a "sunburst" hydride microstructure was determined at 25°C. The results were obtained utilizing a novel testing procedure in which a narrow linear strip of hydride blister was fractured at small loads under bending to create a well-defined sharp precrack that arrested at the blister-substrate interface. The hydriding procedure also forms "sunburst" hydrides emanating from the blister that were aligned both in the plane of the crack and in the crack growth direction. Subsequent tensile loading caused crack growth initiation into the field of "sunburst" hydrides. Specimen failure occurred under near-linear elastic behavior, and the fracture toughness for crack growth initiation into sunburst hydrides was in the range KQ ≈ 10-15 MPa√m. These results, when combined with those of a previous study , indicate that the through-thickness crack growth initiation toughness at 25°C is very sensitive to the hydride microstructure.