We study the coupled Wong Yang-Mills equations for both U(1) and SU(2) gauge groups and anisotropic particle momentum distributions numerically on a lattice. For weak fields with initial energy density much smaller than that of the particles we confirm the existence of plasma instabilities and exponential growth of the fields which has been discussed previously. Also, the SU(2) case is qualitatively similar to U(1), and we do find strong ``abelianization'' of the non-Abelian fields during the period of exponential growth. However, the effect nearly disappears when the fields are strong. This is because of the very rapid isotropization of the particle momenta by deflection in a strong field on time scales comparable to that for the development of Yang-Mills instabilities. This happens without entropy production which makes it very interesting for the phenomenology of high-energy heavy-ion collisions.

version3: 8 pages, 9 figures, revised discussion of entropy production, a few additional references, supplement: 3 figures showing the dependence on the lattice spacing and on the number of test-particles; to appear in PLB