The linear Vlasov theory of the universal instability driven by a density gradient in a unidirectional magnetic field is considered in the slab model with the local approximation. The electrostatic approximation is used, but the full resonant effects of a magnetic field gradient drift are included for the ions. Both electron and ion ∇Beffects reduce the growth rate, but the latter is more important. It is shown that the nonresonant ion ∇Bapproximation fails at relatively low &bgr; and that the conventional resonant ion approximation becomes invalid as &bgr; increases.