[en] The magnetic field pattern in a flat wiggler with iron teeth is discussed. A ''flat'' wiggler is one in which the magnetic fields are all in the vertical direction, alternating in polarity. In the absence of iron, a discussion of such wigglers has been given by J.P. Blewett and R. Chasman. A segment of a flat wiggler is shown. An accurate analytical approximation to the problem for the case when the iron permeability is infinite is presented. Saturation effects in the iron were studied using the POISSON computer program. The analytic approximation in the case of infinite permeability was derived