  An ordinary differential operator of the fourth order with coefficients
converging at infinity sufficiently rapidly to constant limits is considered.
Scattering theory for this operator is developed in terms of special solutions
of the corresponding differential equation. In contrast to equations of second
order "scattering" solutions contain exponentially decaying terms. A relation
between the scattering matrix and a matrix of coefficients at exponentially
decaying modes is found. In the second part of the paper the operator $D^4$ on
the half-axis with different boundary conditions at the point zero is studied.
Explicit formulas for basic objects of the scattering theory are found. In
particular, a classification of different types of zero-energy resonances is
given.
