We study the structure of the phase space for a system of N molecules of ellipsoidal symmetry, as a function of concentration and temperature. A classical lattice gas approximation is considered and a single molecule is described by a rigid ellipsoidal core with weak attractive tails along the long axis. The method adopted is a second-order mean-field approach – designed in such a way as to keep into account the fluctuations from equilibrium of the order parameters up to the fourth order – combined with a cumulant-cluster expansion, and improved by keeping track of the short-range correlations. Preliminary numerical calculations show the existence, in the case of zero attractive tail, of a second order phase transition.