  We consider an exclusion process, with particles injected with rate $\alpha$
at the origin and removed with rate $\beta$ at the right boundary of a
one-dimensional chain of sites. The particles are allowed to hop onto
unoccupied sites, to the right only. For the special case of $\alpha=\beta=1$
the model was solved previously by Derrida et al. Here we extend the solution
to general $\alpha,\beta$. The phase diagram obtained from our exact solution
differs from the one predicted by the mean field approximation.
