The expansion of a plasma slab into a vacuum is studied using one-dimensional and two-dimensional particle-in-cell simulations. As electrons transfer their longitudinal kinetic energy to ions during the expansion, the electron temperature becomes anisotropic. Once this anisotropy exceeds a threshold value, it drives the Weibel instability, leading to magnetic fields in the megagauss range. These fields induce energy transfer between the longitudinal and transverses directions, which influences the expansion. The impact of a cold electron population on this phenomenon is also investigated. Plasma expansion is a fundamental process which occurs in very different fields, such as astrophysics ͓1,2͔, laser-plasma ion acceleration ͓3–5͔ and thin-film deposition ͓6͔. This phenomenon is usually described by simple one-dimensional models ͓7–9͔. Yet, even when the system is translation-invariant along the plasma surface, several effects ͑e.g., Coulomb collisions ͓10͔͒ can induce momentum transfer between the longitudinal and transverse directions. The purely one-dimensional ͑1D͒ description is thus, in general, inaccurate. In this paper, we show that self-generated magnetic fields can lead to such momentum transfer during the expansion of a collisionless plasma slab. This study is of particular interest in the context of laser-plasma ion acceleration , where an intense laser pulse is focused on a thin foil to create a hot electron population that transfers progressively its energy to ions via the ambipolar electric field at the plasma surface ͓11͔. We assume here that the electron distribution is initially Maxwellian with an isotropic temperature. As the plasma expands, the longitudinal temperature T ʈ decreases and the anisotropy parameter A = T Ќ / T ʈ − 1 increases, which eventually leads to the growth of the Weibel instability ͓12–18͔. The most unstable modes are obtained for k = k x e x , where e x is a unit vector normal to the plasma surface. In this case, the maximum unstable wave vector is k x