Communication: Stabilization of magnetic fluids can be achieved by coating the particles, e.g., amorphous cobalt nanoparticles, with long-chain surfactant molecules, e.g., oleic acid. What is the driving mechanism? It is Shown that the particles try to self-organize, growth proceeding until uniform acicular-shaped particles (see Figure) are reached. A mechanism is proposed and the role of the surfactant discussed.