The miscibility gap of the A(x)B(1-x)C ternary alloys is described using the cluster variation method with a tetrahedron as the basic cluster. The maximum temperature of the boundary of miscibility gap is T-t = 0.835 alpha/2R. It is shown that the miscibility gaps of A(x)B(1-x)C ternary alloys with zinc-blende and wurtzite structures coincide in the approximation used.