In this paper, we report for the first time the determination of the real part of the third-order nonlinear susceptibility, x(3) (–ω, ω, 0, 0), measurements in the isotropic phase of a nematic liquid crystal 4ʹ-n-pentyl-4-cyanobiphenyl (KI 5) from electro-optic Kerr effect experiments . The value of x(3) obtained at 632.8 run is found to be 1 .08 × 10-18 m2 V-2 corresponding to a temperature 35. 5 ° C and is the highest so far reported to the best of our knowledge. © 1993 Taylor & Francis Ltd.