  In this paper we analyze the flow of a family of three dimensional
Lotka-Volterra systems restricted to an invariant and bounded region. The
behaviour of the flow in the interior of this region is simple: either every
orbit is a periodic orbit or they move from one boundary to another.
Nevertheless the complete study of the limit sets in the boundary allows to
understand the bifurcations which take place in the region as a global
bifurcation that we denote by focus--center--focus bifurcation.
