a b s t r a c tThe Mathieu operator L(y) = −y ′′ + 2a cos(2x)y, a ∈ C, a ̸ = 0, considered with periodic or anti-periodic boundary conditions has, close to n 2 for large enough n, two periodic (if n is even) or anti-periodic (if n is odd) eigenvalues λ − n , λ + n . For fixed a, we show thatThis result extends the asymptotic formula of Harrell-Avron-Simon by providing more asymptotic terms.