  This Letter reports new results on muon neutrino disappearance from NOvA,
using a 14 kton detector equivalent exposure of $6.05\times10^{20}$
protons-on-target from the NuMI beam at the Fermi National Accelerator
Laboratory. The measurement probes the muon-tau symmetry hypothesis that
requires maximal mixing ($\theta_{23} = \pi/4$). Assuming the normal mass
hierarchy, we find $\Delta m^2 = (2.67 \pm 0.11)\times 10^{-3}$ eV$^2$ and
$\sin^2 \theta_{23}$ at the two statistically degenerate values
$0.404^{+0.030}_{-0.022}$ and $0.624^{+0.022}_{-0.030}$, both at the 68%
confidence level. Our data disfavor the maximal mixing scenario with 2.6
$\sigma$ significance.
