  We evaluate the S-wave pion--nucleon scattering length $a^-$ in the framework
of heavy baryon chiral perturbation theory up--to--and--including terms of
order $M_\pi^4$. We show that the order $M_\pi^4$ piece of the isovector
amplitude at threshold, $T^-_{\rm thr}$, vanishes exactly. We predict for the
isovector scattering length, $0.088 \, M_{\pi^+}^{-1} \le a^- \le 0.096 \,
M_{\pi^+}^{-1}$.
