  We develop a quasiclassical method based on the path integral formalism, to
study the influence of disorder on magnetooscillations of the density of states
and conductivity. The treatment is appropriate for electron systems in the
presence of a random potential with large correlation length or a random
magnetic field, which are characterisitic features of various 2D electronic
systems presently studied in experiment. In particular, we study the system of
composite fermions in the fractional quantum Hall effect device, which are
coupled to the Chern--Simons field and subject to a long--range random
potential.
