Langmuir-Blodgett films obtained by successive deposition of a known number of fatty-acid monolayers are generally assumed to be uniaxial with in-plane isotropy. In order to test the validity of this assumption, we measure the generalized ellipsometric response of such quasi-bidimensional molecular systems as a function of their azimuthal orientation theta relative to the incident plane. These measurements which are related to the nan-diagonal terms of the Jones reflection matrix provide information on the orientation of the principal axes and the principal indices of the dielectric tensor of the film. Using a multilayer anisotropic model based on the Berreman formalism, regression of the experimental data gives the following results. The studied LB films are biaxial with one of the principal axis tilted at 1.5 degrees with respect to the normal of the film and the in-plane birefringence varies from 2 X 10(-2) to 1 X 10(-2) over the 0.24-0.6 mu m spectral domain. This in-plane anisotropy is connected to the withdrawing direction of the substrate during the LB film deposition.