PACS. 05.20.Jj -Statistical mechanics of classical fluids. PACS. 12.20.-m -Quantum electrodynamics. PACS. 11.10.Wx -Finite-temperature field theory.Abstract. -The high-temperature aspects of the Casimir force between two neutral conducting walls are studied. The mathematical model of "inert" ideal-conductor walls, considered in the original formulations of the Casimir effect, is based on the universal properties of the electromagnetic radiation in the vacuum between the conductors, with zero boundary conditions for the tangential components of the electric field on the walls. This formulation seems to be in agreement with experiments on metallic conductors at room temperature. At high temperatures or large distances, at least, fluctuations of the electric field are present in the bulk and at the surface of a particle system forming the walls, even in the high-density limit: "living" ideal conductors. This makes the enforcement of the inert boundary conditions inadequate. Within a hierarchy of length scales, the high-temperature Casimir force is shown to be entirely determined by the thermal fluctuations in the conducting walls, modelled microscopically by classical Coulomb fluids in the Debye-Hückel regime. The semi-classical regime, in the framework of quantum electrodynamics is studied in the companion letter by P.R.Buenzli and Ph.A.Martin [1].This letter is related to the one by Buenzli and Martin [1]. For the sake of completeness, we cannot avoid repeating a few things.Casimir showed in his famous paper [2] that fluctuations of the electromagnetic field in vacuum can be detected and quantitatively estimated via the measurement of a macroscopic attractive force between two parallel neutral metallic plates; for a nice introduction to the Casimir effect see [3] and for an exhaustive review see [4].Let us recall briefly, within the formalism of Ref.[3], some aspects of the usual theory for plates considered as made of ideal conductors, which are relevant in view of the present letter. We consider the 3D Cartesian space of points r = (x, y, z) where a vacuum is localized in the subspace Λ = {r|x ∈ (−d/2, d/2); (y, z) ∈ R 2 } between two ideal-conductor walls (thick slabs) at a distance d from each other. The time-dependent electric E(r, t) and magnetic B(r, t) fields in Λ are the solutions of the Maxwell equations in vacuum, subject to the boundary