We offer an effective medium model for the elastic moduli of high-porosity ocean-bottom sediments. The elastic constants of the dry-sediment frame depend on porosity, elastic moduli of the solid phase, and effective pressure. The model connects two end points in the elastic-modulus-porosity plane: the Hertz-Mindlin modulus of a dense elastic sphere pack at critical porosity; and zero at 100% porosity. The elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. Unlike the suspension model, our model assigns non-zero elastic constants to the dry-sediment frame and can predict the shear-wave velocity. Unlike various modifications of the travel-time-average equation, it is first-principle-based and contains only physical parameters. We justify this model by matching sonic data in shallow marine sediments and in an ODP well. ¿ 1999 American Geophysical Union