A one‐dimensional electromagnetic particle‐in‐cell (PIC) model is used to investigate the Alfve´n ion‐cyclotron (AIC) instability in high‐&bgr; plasmas with large ion temperature anisotropies. The simulation results are compared with linear and quasilinear theory. Nonlinear effects, such as the rate at which the anisotropy relaxes and the level of perturbed magnetic wave energy at saturation, are compared as a function of &bgr; and of the initial ion anisotropy. It is found that for large &bgr;⊥(&bgr;⊥>3) the anisotropy decays to a residual level in a time comparable to an ion‐cyclotron period. Simulations designed to drive the instability by replenishing particles from an anisotropic temperature reservoir are also discussed.