We provide a criterion in order to decide the stability of non-degenerate equilibrium states of completely integrable systems. More precisely, given a HamiltonPoisson realization of a completely integrable system generated by a smooth n− dimensional vector field, X, and a non-degenerate regular (in the Poisson sense) equilibrium state, x e , we define a scalar quantity, I X (x e ), whose sign determines the stability of the equilibrium. Moreover, if I X (x e ) > 0, then around x e , there exist one-parameter families of periodic orbits shrinking to {x e }, whose periods approach 2π/ I X (x e ) as the parameter goes to zero. The theoretical results are illustrated in the case of the Rikitake dynamical system.