When a chaotic attractor lies in an invariant subspace, as in systems
with symmetry, riddling can occur. Riddling refers to the situation wh
ere the basin of a chaotic attractor is riddled with holes that belong
to the basin of another attractor. We establish properties of the rid
dling bifurcation that occurs when an unstable periodic orbit embedded
in the chaotic attractor, usually of low period, becomes transversely
unstable. An immediate physical consequence of the riddling bifurcati
on is that an extraordinarily low fraction of the trajectories in the
invariant subspace diverge when there is a symmetry breaking.