Single-crystal Brillouin scattering measurements of elastic moduli were performed on an eclogitic omphacite (space group C2/c) with chemical composition close to the diopside (CaMgSi2O6. 29.9 mol %)-jadeite (NaAlSi2O6, 57.8 mol %) join. From the 13 elastic moduli measured under ambient conditions we calculate the adiabatic bulk modulus, Ks=130.8¿0.5 GPa, and the shear modulus, μ=79.2¿0.2 GPa. The magnitudes of all longitudinal elastic moduli (c11, c22, and c33) are inferred to be influenced primarily by the MgO6 and CaO8, or M polyhedra. Although the silica tetrahedra are extremely stiff, the great rotational freedom of these polyhedra yields a mechanical unit (tetrahedral chains) which is relatively compliant. Virtually all of the omphacite moduli are intermediate in value to those for diopside and jadeite. Most of the moduli for omphacitic pyroxene can be estimated to within several percent by interpolation between the end member elastic moduli weighted according to the molar percentages of diopside and jadeite. ¿ American Geophysical Union 1992