The structure of thin Fe overlayers on the Co(1 1 2 0) surface has been investigated by low energy electron diffraction (LEED) and by primary-beam diffraction modulated electron emission (PDMEE), as a function of the Fe coverage (6-50 ML) and of the substrate temperature (between room temperature and 300 degreesC). The structure of the layers was the tetragonally distorted bet modification of the equilibrium bcc phase with the (0 0 1)Fe//(1 1 2 0)Co and [1 1 0]Fe//[1 1 0 0]Co relationships with the Co substrate. The Fe(0 0 1)-Co(1 1 2 0) system exhibits a very large misfit asymmetry in the interface plane. As a consequence, a breakdown of the long-range order occurs in the Fe film along the [1 1 0 0] substrate direction.