  Rayleigh-Taylor instability in a compressible fluid is reconsidered. The
density is allowed to vary with pressure under the barotropy assumption. For
the case with equal speeds of sound in the two superposed fluids, in order to
give a non-trivial compressibility correction to the Rayleigh-Taylor growth
rate, the compressibility correction is calculated to $O(g^2/k^2a^4)$. To this
order, compressibility effects are found to reduce the growth rate.
