  This paper has been withdrawn by the authors. A class of pseudodifferential
operators on the Heisenberg group is defined. As it should be, this class is an
algebra containing the class of differential operators. Furthermore, those
pseudodifferential operators act continuously on Sobolev spaces and the loss of
derivatives may be controled by the order of the operator. Although a large
number of works have been devoted in the past to the construction and the study
of algebras of variable-coefficient operators, including some very interesting
works on the Heisenberg group, our approach is different, and in particular
puts into light microlocal directions and completes, with the Littlewood-Paley
theory developed in \cite{bgx} and \cite{bg}, a microlocal analysis of the
Heisenberg group.
