  We present the results of three-dimensional numerical simulations that
include the effects of hydrodynamical forces and gas drag upon an evolving
dusty gas disk. We briefly describe a new parallel, two phase numerical code
based upon the smoothed particle hydrodynamics (SPH) technique in which the gas
and dust phases are represented by two distinct types of particles. We use the
code to follow the dynamical evolution of a population of grains in a gaseous
protoplanetary disk in order to understand the distribution of grains of
different sizes within the disk. Our ``grains'' range from metre to
submillimetre in size.
