A microcanonical algorithm based upon a deterministic cellular automaton had been presented by Creutz for simulating the Ising system. This model is tested on a SIMD parallel computer working as a cellular automaton and found to fail at low temperature (T ≤ 0.91T_c). Two improved versions, a random one and a deterministic one, are presented and found to work at low temperature. The deterministic version is successfully tested against the Metropolis algorithm and gives a rather good estimation for the critical exponent β in a 48 × 48 square lattice.