  We investigate the stability of cosmological scaling solutions describing a
barotropic fluid with $p=(\gamma-1)\rho$ and a non-interacting scalar field
$\phi$ with an exponential potential $V(\phi)=V_0\e^{-\kappa\phi}$. We study
homogeneous and isotropic spacetimes with non-zero spatial curvature and find
three possible asymptotic future attractors in an ever-expanding universe. One
is the zero-curvature power-law inflation solution where $\Omega_\phi=1$
($\gamma<2/3,\kappa^2<3\gamma$ and $\gamma>2/3,\kappa^2<2$). Another is the
zero-curvature scaling solution, first identified by Wetterich, where the
energy density of the scalar field is proportional to that of matter with
$\Omega_\phi=3\gamma/\kappa^2$ ($\gamma<2/3,\kappa^2>3\gamma$). We find that
this matter scaling solution is unstable to curvature perturbations for
$\gamma>2/3$. The third possible future asymptotic attractor is a solution with
negative spatial curvature where the scalar field energy density remains
proportional to the curvature with $\Omega_\phi=2/\kappa^2$
($\gamma>2/3,\kappa^2>2$). We find that solutions with $\Omega_\phi=0$ are
never late-time attractors.
