The dc Kerr response and the pre-transitional behaviour of a liquid crystalline isothiocyanate (6HBT) at and above the nematic-isotropic phase transition temperature are reported. The Kerr law has been verified and the real part of the third order nonlinear susceptibilities (χ(3)) above the nematic - isotropic transition are determined for the liquid crystalline isothiocyanate from the electro-optic Kerr effect (EOKE) experiments. The variation of the Kerr constant with temperature has been studied on the basis of the Landau-deGennes model of nematic - isotropic phase transition. The variation of susceptibility with temperature has also been studied.