We performed scanning tunneling microscope experiments with GaAs tips under illumination by circularly polarized light in ultrahigh vacuum at room temperature. Ferromagnetic ultrathin cobalt layers with perpendicular magnetization were employed as test samples. The spin-polarized tunneling current was shown to exhibit a magnetic sensitive component, changing its sign from one place to another on the sample surface. Reproducible images exhibiting magnetic domains of a 300–500 nm characteristic size were obtained. These results agree with magnetic force microscope measurements performed on a typical sample. ©1997 American Institute of Physics.