Abstract. Let X be a compact connected Kähler-Einstein manifold with c 1 (T X) ≥ 0. If there is a semistable Higgs vector bundle (E , θ) on X with θ = 0, then we show that c 1 (T X) = 0; any X satisfying this condition is called a Calabi-Yau manifold, and it admits a Ricci-flat Kähler form [Ya]. Let (E , θ) be a polystable Higgs vector bundle on a compact Ricci-flat Kähler manifold X. Let h be an Hermitian structure on E satisfying the Yang-Mills-Higgs equation for (E , θ). We prove that h also satisfies the Yang-Mills-Higgs equation for (E , 0). A similar result is proved for Hermitian structures on principal Higgs bundles on X satisfying the Yang-Mills-Higgs equation.