The internal electric field distribution has been measured in photoconducting CdS single crystals using the linear electro‐optic effect. The experimental results show a nearly homogeneous field distribution in the crystals for applied fields below the threshold for acoustoelectric oscillations, while a high‐field region was created near the anode for fields above the threshold field. The reproducibility from sample to sample was rather poor. The linear electro‐optic constant was calculated from the experimental results: (r′113−r′333)=2.7×10−12m/V at the optical wavelength 6328 Å.