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The interaction between superconductivity and ferromagnetism, which entails incompatible spin order, is one of the problems of fundamental interest in condensed-matter physics. In general, when a ferromagnet is placed in contact with a superconductor, the Cooper pairs from the superconductor are not expected to survive beyond at most a few nanometres into the ferromagnet. Here we present a systematic study of single-crystal ferromagnetic cobalt nanowires sandwiched between superconducting electrodes. Surprisingly, we find that a cobalt wire as long as 600 nm attains zero resistance at low temperatures. For even longer nanowires, the transition to incomplete superconductivity is foreshadowed by a strikingly large and sharp resistance peak near the superconducting transition temperature of the electrodes. Although the origin of the ‘critical peak’ remains mysterious, our analysis strongly points against …