The growth of CoC(2)O(4)(.)2H(2)O particles during aqueous precipitation has been followed by several electron microscopy techniques. A multiple-stage growth leading to final particles with a core-shell structure has been observed. In the first stage, the primary particles grow and aggregate to form secondary particles which in turn agglomerate to form the elongated core of the final particles. The final particle crystal faces are formed around the core, layer by layer and face by face, by primary particle self-assembly. The core is disordered and composed of both isolated partially crystalline primary particles and aggregated primary particles. The shell faces are ordered and made up by assembly of strings of 5-7 nm crystalline nanodomains.