  This paper reports the first measurement using the NOvA detectors of
$\nu_\mu$ disappearance in a $\nu_\mu$ beam. The analysis uses a 14
kton-equivalent exposure of $2.74 \times 10^{20}$ protons-on-target from the
Fermilab NuMI beam. Assuming the normal neutrino mass hierarchy, we measure
$\Delta m^{2}_{32}=(2.52^{+0.20}_{-0.18})\times 10^{-3}$ eV$^{2}$ and
$\sin^2\theta_{23}$ in the range 0.38-0.65, both at the 68% confidence level,
with two statistically-degenerate best fit points at $\sin^2\theta_{23} = $
0.43 and 0.60. Results for the inverted mass hierarchy are also presented.
