We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross‐tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross‐tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross‐tie wall. ©1996 American Institute of Physics.