We have applied magnetic force microscopy (MFM) with an in situ electromagnet to study the switching of the magnetization of submicrometer Co dots fabricated by means of electron-beam lithography. By using the MFM tip as a local-field source, the magnetization of individual single-domain Co dots could be reversed. Micromagnetic simulations show that the switching process is induced by the stray field of the MFM tip. Furthermore, the external field that is necessary to support switching of the dot depends on the tip-dot separation.