The elasto‐optic matrix elements for longitudinal waves in four new single crystals have been measured. Piezoelectric bismuth silicon oxide and bismuth germanium oxide, cubic point group 23, show an elasto‐optic strength of 6.0 and 6.6, respectively, relative to fused silica based on an equal acoustic power criterion discussed below. Ferroelectric crystals from the strontium barium niobate system, tetragonal point group 4mm, were also studied. One with a Sr to Ba ratio of 3:1 showed an elasto‐optic strength of 26; another with a ratio of 1:1 had a strength of 5.8.