  Cellular automata, CA for short are continuous maps defined on the set of
configurations over a finite alphabet A that commutes with the shift. They are
characterized by the existence of local function which determine by local
behavior the image of an element of the configurations space. We will study
periodic factors of cellular automata. We classify periodic factors according
to their periods. We show that for a surjective cellular automaton with
equicontinuity points but without being equicontinuous there is an infinity of
classes of equivalence of periodic factors
