  We investigate the decays $D^0\to\pi^-\ell^+\nu$ and $D^0\to K^-\ell^+ \nu$,
where $\ell$ is $e$ or $\mu$, using approximately 7 ${\rm fb}^{-1}$ of data
collected with the CLEO III detector. We find $R_0\equiv {\cal B}(D^0\to
\pi^-e^+\nu)/{\cal B}(D^0\to K^-e^+\nu)= 0.082 \pm 0.006 \pm 0.005$. Fits to
the kinematic distributions of the data provide parameters describing the form
factor of each mode. Combining the form factor results and $R_0$ gives
$|f^{\pi}_{+}(0)|^2 |V_{cd}|^2/|f^K_{+}(0)|^2 |V_{cs}|^2 =
0.038^{+0.006+0.005}_{-0.007-0.003}$.
