  By supplementing minimal supersymmetric SU(5) (MSSU(5)) with a flavor ${\cal
U}(1)$ symmetry and two pairs of $\bar{15}+15$ `matter' supermultiplets, we
present an improved model which explains the charged fermion mass hierarchies
and the magnitudes of the CKM matrix elements, while avoiding the undesirable
asymptotic mass relations $m_s=m_{\mu}, m_d/m_s = m_e/m_{\mu}$. The strong
coupling $\alpha_s (M_Z)$ is predicted to be approximately 0.115, and the
proton lifetime is estimated to be about five times larger than the MSSU(5)
value. The atmospheric and solar neutrino puzzles are respectively resolved via
maximal $\nu_{\mu}-\nu_{\tau}$ and small mixing angle $\nu_e-\nu_s$ MSW
oscillations, where $\nu_s$ denotes a sterile neutrino. The ${\cal U}(1)$
symmetry ensures not only a light $\nu_s$ but also automatic `matter' parity.
