The FCC Heisenberg spin lattice with AF NN interactions is a known model of an inherently frustrated system. Due to the continuous ground state degeneracy, the spin order in this lattice is particularly sensitive to weak symmetry-breaking perturbations, such as Dzyaloshinski–Moriya (D-M) interactions or anisotropic strains. These effects may create a rich variety of colinear and noncolinear structures. The limited availability of naturally existing prototypes has resulted in little experimental work in this field. However, the emergence of novel epitaxial structures containing zinc-blende type MnSe and MnTe, which are very close approximations of the NN FCC Heisenberg model, opens new opportunities for such studies. In many of these systems the MnSe and MnTe layers are strongly strained (c/a∼0.95–1.05) due to lattice mismatch effects. The resulting anisotropy in the Mn-Mn exchange may lead to a dramatic change in the spin structure—e.g., studies of MnSe/ZnTe superlattice systems have revealed a transition from a type III AF order to an entirely new incommensurate helical state not yet seen in any FCC aniferromagnet.1 In this paper we report neutron diffraction data from ‘‘thick’’ (∼1 μm) Co0.25Mn0.75Se films. Detailed mapping of the magnetic diffraction intensity reveals a shift from perfect type III lattice points, showing that detectable incommensurate effects may be produced even by weak residual stresses. Our results point out that noncolinear effects arising from strains may be indistinguishable from canting produced by D-M exchange,2 which may additionally complicate the detection of this weak interaction in Mn chalcogenides.