A technique is presented for measuring the “shear” photoelastic constants of lithium niobate by Bragg scattering from acoustic shear waves at moderate frequencies and at small Bragg angles. Values of P65=0.048, P14=0.056, P44=0.062, and P41 =0.050 are obtained, the first two agreeing well with the results of other workers. Copyright © 1969 by The Institute of Electrical and Electronics Engineers, Inc.