  We present a high accuracy Monte Carlo simulation study of the Isotropic -
Nematic phase transition of a lattice dispersion model of biaxial liquid
crystals. The NI coexistence curve terminating at the Landau critical point
have been determined using multiple histogram reweighting technique. A close
investigation reveals a sharp departure in the nature of the $N$-$I$
coexistence curve in temperature- biaxiality parameter phase diagram in
comparison to the earlier theoretical (either mean-field or computer
simulation) predictions.The coexitence curve shows a change in curvature with
increasing value of the degree molecular biaxiality.
