A single-step method to synthesize monodisperse metallic cobalt particles of diameter around 8 nm is described. The particles are sterically stabilized by modified polyisobutene and form stable ferrofluids in toluene. The core-shell cobalt spheres have a narrow size distribution and are nonaggregated. Various techniques, such as infrared spectroscopy, X-ray diffraction, and elemental analysis establishes the presence of grafted modified polyisobutene on the surface of cobalt particles. The hydrodynamic thickness of the grafted polyisobutene calculated from sedimentation and viscosity measurements and independently from the dynamic light scattering is ca. 10 mn. The dependence of magnetization has been measured for ferrofluid, showing superparamagnetic behavior above 247 K. Due to anisotropy, superposition of magnetic curves is not observed below this temperature. The concentration dependence of the sedimentation coefficient agrees qualitatively with the theory for dipolar spheres. SAXS data on concentrated ferrofluid dispersion indicate interpenetration of polyisobutene chains grafted to the cobalt particles.