  Floyd languages (FL), alias Operator Precedence Languages, have recently
received renewed attention thanks to their closure properties and local
parsability which allow one to apply automatic verification techniques (e.g.
model checking) and parallel and incremental parsing. They properly include
various other classes, noticeably Visual Pushdown languages. In this paper we
provide a characterization of FL in terms a monadic second order logic (MSO),
in the same style as Buchi's one for regular languages. We prove the
equivalence between automata recognizing FL and the MSO formalization.
