Lattice dynamical properties of antiferromagnetic f. c. c. Mn83Ni11C6 and Mn85Ni9C6 have been studied by means of ultrasonics and neutron scattering. The two alloys illustrate how, with increase of manganese content across this region of the constitution diagram, severe elastic anomalies develop in these materials. In particular, for these concentrations the magnitude of the elastic stiffness constant C11 begins to fall off drastically towards the lower temperatures, passing below C44; in Mn85Ni9C6 it reduces with reducing temperature to such an extent that a cubic-to-tetragonal martensitic transformation is induced. At the transformation temperature C11 has a minimum, rising again thereafter. A detailed study has been made of one specimen of Mn85Ni9C6 that showed a nearly second-order structure transition. In this sample, 1/2(C11 — C12) softens linearly with temperature, reaching 0.27 x 1010 N m-2 ( x 1011 dyn cm-2) at the transformation temperature Tm = 174 ± 2 K, where Zener’s anisotropy parameter attains the value 36. Other elastic properties likewise prove to be extremely unusual, with longitudinal sound propagating more slowly than transverse sound along the cube axis directions. Phonon energy measurements, made to an accuracy of about 40 μeV, show however that as a function of wavevector appreciable phonon softening is restricted to a small central region of the Brillouin zone within about a tenth of a reciprocal lattice unit of the origin. The anomalous inter-atomic forces responsible are markedly non-central, and it is argued they are a consequence of an enhanced electron-phonon interaction arising because the manganese moment is unstable. The applicability of molecular field theory to the transition is well confirmed, the critical index γ of the order parameter above Tm being obtained as 1.03 ± 0.07.