This paper presents an investigation of the value of soil density that controls the velocity of small strain shear waves in saturated soil. An "effective soil density" is defined that is related to the fraction of pore water that moves with the solid skeleton during shear wave propagation. Biot theory indicates that the ratio of effective soil density to saturated soil density is always ≤ 1 and is a function of the specific gravity of solids, porosity, hydraulic conductivity, and shear wave frequency. Except for extreme cases, the effective density ratio will range from 0.75 to 1.0. For many geotechnical applications, effective soil density will be equal to saturated soil density for low hydraulic conductivity materials (clays and silts). On the other hand, consideration of effective soil density may be important for high hydraulic conductivity materials (clean sands and gravels) in some cases. The findings are relevant to applications involving the propagation of small strain shear waves through saturated soil, and in particular for laboratory and field tests in which shear modulus is back-calculated from measured shear wave velocity.