Fatigue behaviour at room temperature and its influence on superconducting current and upper critical magnetic field at 4.2 k were studied using multifilamentary composite wire with a copper ratio of 1.55 and an overall diameter of 0.812 mm in which 241 filaments are embedded. The filament itself was not fractured by the fatigue when tested separately. In the composite, the fatigue cracks nucleated in the copper, which grew and caused the fracture of the filaments. When the maximum stress in the fatigue test was low, one crack among many grew preferentially; when the maximum stress was intermediate, many cracks could grow at different cross-sections before the overall fracture of the composite; when the maximum stress was high, the composite was fractured before large fatigue crack growth. Because of such a maximum stress dependence of the growth of the fatigue damage, the critical current measured using the segments of the fatigue-fractured specimens for the intermediate maximum stress range was lower than that for the low and high maximum stress ranges. The mode I fracture toughness of the present composite wire, estimated from the size of the strength-determining fatigue crack in combination with the maximum stress, was around .