  We propose a dynamical theory of the stripe phase arising in a
two-dimensional electron liquid near half-integral fillings of high Landau
levels. The system is modelled as a novel type of a smectic liquid crystal with
the Lorentz force dominated dynamics. We calculate the structure factor, the
dispersion relation of the collective modes (magnetophonons), and their
intrinsic attenuation rate. We find strong power-law renormalizations of the
elastic and dissipative coefficients by thermal fluctuations familiar from the
conventional smectics but with different dynamical scaling exponents.
