  We derive the metric for a Bianchi type I space-time with energy density that
is dominated by that of a perfect fluid with equation of state $p=w\rho$ and
whose anisotropy is seeded by a fixed norm spacelike vector field. We solve for
the evolution of perturbations about this space-time. In particular, the Jeans
instability in an expanding flat Friedmann-Robertson-Walker universe is
modified by the presence of the vector field so that energy density
perturbations develop direction-dependent growth. We also briefly consider
observational limits on the vector field vacuum expectation value, $m$. We find
that, if $m$ is constant during recombination and thereafter, $m \lesssim
10^{14} GeV$.
