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Magnetic cobalt chains, self-assembled by microspheres of hexagonal-phase cobalt, have been synthesized at 100 degrees C via a hydrothermal reduction route in the presence of cobalt chloride, the surfactant sodium dodecylsulfate (SDS) (or cetyltrimethylammonium bromide CTAB) and the complex reagent sodium tartrate. As-synthesized, the chains are 100-300 mu m in length and the cobalt microspheres, which consist of nanosheets with an average thickness of about 60 nm, are 5-10 mu m in diameter. The magnetic hysteresis loops at 5 K and 300 K of the chains of microspheres show ferromagnetic characteristics. The morphologies of the microspheres can be controlled by adjusting the concentrations of the surfactant and the complex reagent and also the reaction temperature. (C) 2007 Elsevier Ltd. All rights reserved.