We report the observation of the Kerr effect in a streamer head in experiments with electrical breakdown of water. The propagation of the streamer in water is associated with localized enhancement of the electric field surrounding the streamer head. Polarizational measurements show that the electric field around the streamer head is estimated to be 1.7-2.2 MV/cm, which is approximately six times higher than the maximum interelectrode electric field of 0.37 MV/cm. Electrostatic simulations support the experimental data. (C) 2006 American Institute of Physics.