Electric field induced isotropic-nematic-smectic A phase transitions, Kerr effect and fluctuations. Under a strong electric field, dielectric anisotropy variations have two main contribution: (i) a linear one originating from quenching of the nematic director fluctuations and (ii) a quadratic contribution from the Kerr effect. The dependence on temperature and field, and the role of these contributions on the : isotropic--nematic, isotropic—smectic, and nematic--smectic-A transitions are analysed by birefringence measurements under strong electric fields up to 5 105 V/cm. The critical or tricritical points of the transitions are localized, non spontaneous phases are induced and experimental evidence of the Halperin-Lubensky-Ma effect has been observed. 1. 2. 3. 4. 35. B.I. Halperin, T.C. Lubensky and S.K. Ma, Phys. Rev. Lett. 32, 292 (1974). Y. Poggi and J.C. Filippini, Phys. Rev. Lett. 39, 150 (1977). I. Lelidis and G. Durand, Phys. Rev. Lett. 73, 672 (1994). R. Mukhopadhyay, A. Yethiraj and J. Bechhoefer, Phys. Rev. Lett. 83, 4796 (1999). I. Lelidis, Phys. Rev. Lett. 86, 1267 (2001).