An experimental study of neutron self-shielding in cobalt foils and wires has been made by an activation technique. Cobalt foils ranging in thickness from 0.0004 to 0.004 in. and 0.005 in. thick Co-Al alloy reference foils containing 1% Co were irradiated under cadmium in the NRX reactor. The Co60 specific activity in the pure foils was compared with that in the reference foils to give the gross self-shielding factors. Experimental resonance self-shielding factors were obtained by separating the 1/v part from the gross self-shielding factors, and agreement with calculated resonance self-shielding factors for foils was observed. Self-shielding factors for wire ranging in diameter from 0.001 to 0.025 in. and for 0.050 in. diam 1 % Co-Al alloy wire were also measured since they are of practical importance and have not been calculated. Measurements of self-shielding factors for thermal neutrons were made for cobalt wires and these agree with theory.