  We show that the prediction for $\alpha_s(M_Z)$ in flipped SU(5) is naturally
lower than in minimal SU(5), and that the former can accommodate the full range
of $\alpha_s(M_Z)$ presently allowed by experiment. Our computations include
two-loop ($\delta_{\rm 2loop}$) and light ($\delta_{\rm light}$) and heavy
($\delta_{\rm heavy}$) threshold effects. Unlike minimal SU(5), in flipped
SU(5) the heavy threshold effects can naturally decrease the predicted value of
$\alpha_s(M_Z)$. We also show that the value of the proton lifetime into the
dominant channel $p\to e^+\pi^0$ is within the observable range at
SuperKamiokande, and should discriminate against minimal supersymmetric SU(5),
where the dominant mode is $p\to\bar\nu K^+$.
