Using a Beckman spectrophotometer, light transmission measurements were made on 12 polydisperse butadiene-styrene latices and one essentially monodisperse polystyrene latex. Cell lengths of 1, 5, and 10 cm. were used, and the wave length range covered was 3500–10,000 A. in 500-A. intervals. By employing total polymer content, c, rather than total solids, ratios of turbidity, τ, to c at c = 0, i.e., ( τ/ c) 0, were obtained which were independent of the cell length at each wave length. Application of Mie theory to the measurements yielded particle diameters showing no regular variation with wave length and in good accord with the weight-average diameters of the latices obtained by electron microscopy. The validity of the average was checked by differential creaming of the latices and also by calculation of the averages to be expected from the latex particle size distributions on summation of the scattering coefficients. The range of diameters covered by the data was α = πD w / λ m = 0.30 to 4.74, where D W is the weight-average diameter and λ m the wave length of light in the medium. For light of 10,000 A. in vacuo, the upper limit of α corresponds to D w = 11,400 A.