We consider a mean field theory for nematic liquid crystals consisting of biaxial molecules. The internal degrees of freedom of the molecules are described by eight variables, three of which are the components of the angular momentum. The five remaining variables are the components of the dynamical quadrupole moment. Using the fact that these variables are the generators of SU(3), the partition function in the mean field approximation can be calculated exactly. From a numerical determination of the minimum of the free energy we then show that with decreasing temperature the fluid will have two successive transitions according to the scheme: isotropic → unaxial order → biaxial order. The order parameters and the specific heat are calculated in their dependence on the temperature and on the form of the molecules.