  The multiplicity of charged particles in the central rapidity region has been
measured by the NA57 experiment in Pb--Pb collisions at the CERN SPS at two
beam momenta: 158 A GeV/{\it c} and 40 A GeV/{\it c}. The value of
$dN_{ch}/d\eta$ at the maximum has been determined and its behaviour as a
function of centrality has been studied in the centrality range covered by NA57
(about 50% of the inelastic cross section). The multiplicity increases
approximately logarithmically with the centre of mass energy.
