The effect of magnetic fields on the magnetic domain structure in thin iron films (∼200 A˚) has been studiedinsituby Lorentz microscopy in an electrostatic transmission electron microscope (TEM). The electrostatic instrument provides a specimen region free of magnetic fields. A special solenoid was placed in the specimen region, coaxial with the optic axis. By tilting the specimen holder a known in‐plane component of the field could be applied to the specimen. Studies were made with the in‐plane field applied parallel to and perpendicular to the domain walls.