  Using a genealogically ordered infinite regular language, we know how to
represent an interval of R. Numbers having an ultimately periodic
representation play a special role in classical numeration systems. The aim of
this paper is to characterize the numbers having an ultimately periodic
representation in generalized systems built on a regular language. The
syntactical properties of these words are also investigated. Finally, we show
the equivalence of the classical "theta"-expansions with our generalized
representations in some special case related to a Pisot number "theta".
