We study changes in the chaotic properties of a many-body system undergoing a solid-fluid phase transition. To do this, we compute the temperature dependence of the largest Lyapunov exponents lambda(max) for both two-and three-dimensional periodic systems of N-particles for various densities. The particles interact through a soft-core potential. The two-dimensional system exhibits an apparent second-order phase transition as indicated by a lambda-shaped peak in the specific heat. The first derivative of lambda(max) with respect to the temperature shows a peak at the same temperature. The three-dimensional system shows jumps, in both system energy and lambda(max), at the same temperature, suggesting a first-order phase transition. Relaxation phenomena in the phase-transition region are analyzed by using the local time averages. (C) 1997 American Institute of Physics.