By means of the Hamiltonian formalism, it is shown that the nonlinear Alfv¿n waves in a multispecies plasma in the presence of a driver can be chaotic provided the amplitude of the driver exceeds a certain threshold. This threshold is larger in the presence of heavier ions, e.g., oxygen in the case of comets and helium in the case of solar wind, than for plasma with only two species. This clearly shows that heavier ions tend to reduce the chaos. ¿ American Geophysical Union 1992