00-01E

A microscopic approach to the investigation of the behaviour of a symmetrical binary fluid mixture in the vicinity of the vapour-liquid critical point is proposed. It is shown that the problem can be reduced to the calculation of the partition function of a 3D Ising model in an external field. For a square-well symmetrical binary mixture we calculate the parameters of the critical point (critical temperature and critical density) as functions of the microscopic parameters: the parameter $r$ measuring the relative strength of interactions between the particles of dissimilar and similar species and the parameter $\lambda $ measuring the width of the potential well. The obtained results agree well with the ones of computer simulations.