The optical behaviour of liquid-crystalline 'Blue Phases' (BP) of polar cholesterogenic compounds in electric Fields up to 100 kV • cm-1 has been investigated by means of polarization microscopy. By an observation parallel to the field direction a birefringence dependent on the Field strength was detected in the BP I. This result is discussed in terms of a non-linear electrooptic effect, showing that the BP I becomes optically biaxial in an electric field. In the BP II a birefringence was not observed under the same conditions probably indicating that the field induces an optical uniaxiality with the optic axis parallel to the field. The results are discussed with respect to possible cubic structures of the BPs