  Ion-acoustic modes propagating in unmagnetized dusty plasmas are studied by
applying a generic collisionless fluid model. An Eulerian-to-Lagrangean
variable transformation leads to a new system of evolution equations, which may
be combined into a single (rather complex) equation for the mean ion density.
The linear and weakly nonlinear oscillations regimes are studied and their
modulational stability is investigated. This study extends known previous
results on the Lagrangian description of electron- or ion-acoustic
electrostatic plasma modes. The relevance with the description of dust-ion
acoustic waves propagating in a dusty plasma is also discussed.
