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Results on the magnetic-field dependence of the order parameter in a nematic single crystal are reported for the first time; The induced birefringence of a nematic single crystal of 4 / -«-heptyl-4-cyanobiphenyl was found to vary linearly with the magnetic field as predicted by the fluctuation theory.In this Letter, we report the first measurements of the dependence of the order parameter on magnetic field in a nematic single crystal. These measurements confirm the validity of de Gennes' theory for orientation fluctuations.The most fundamental idea which helps elucidate the behavior of a system near a critical point is the concept that the transition may be described by an order parameter S which represents a numerical measure of the amount and kind of ordering built up in the neighborhood of the critical point. 1 For example, in a single-domain ferromagnetic crystal, with an easy axis of magnetization along the z direction, a suitable order parameter is the statistically averaged z component of the magnetization M. Ferromagnetic critical phenomena have been largely studied and the dependence of the order parameter on the temperature T or on the magnetic field H near the Curie point T c has long been known: Whereas M is proportional to H above T c in the paramagnetic phase, M has a H l/6 variation (with 0 =4 to 5) for T~T C .In the nematic phase of a liquid crystal, the order parameter can be studied by measurement of the diamagnetic anisotropy x a > ov the dielectric anisotropy e a , or the birefringence An. 2 ' 7