Kurtz and Robinson1 have recently proposed a physical model of the electro-optic effect. On its basis they have derived expressions for the electro-optic coefficients of centrosymmetric and non-centrosymmetric crystals. They have also established a relationship between the linear and the quadratic electro-optic coefficients in ferroelectric materials in terms of spontaneous polarization. According to this model the linear electro-optic coefficient vanishes for centrosymmetric crystals. This conforms with the general concept. The purpose of this communication is to point out that a small linear electro-optic effect is also expected to occur in centrosymmetric crystals. Kurtz and Robinson's model1 is modified with respect to this point and a rough theoretical estimate for linear electro-optic coefficients of such crystals is obtained.