  The DONuT experiment collected data in 1997 and published first results in
2000 based on four observed $\nu_\tau$ charged-current (CC) interactions. The
final analysis of the data collected in the experiment is presented in this
paper, based on $3.6 \times 10^{17}$ protons on target using the 800 GeV
Tevatron beam at Fermilab. The number of observed $\nu_\tau$ CC interactions is
9, from a total of 578 observed neutrino interactions. We calculated the
energy-independent part of the tau-neutrino CC cross section ($\nu + \bar
\nu$), relative to the well-known $\nu_e$ and $\nu_\mu$ cross sections. The
ratio $\sigma(\nu_\tau)$/$\sigma(\nu_{e,\mu})$ was found to be
$1.37\pm0.35\pm0.77$. The $\nu_\tau$ CC cross section was found to be $0.72 \pm
0.24\pm0.36 \times 10^{-38}$ cm$^{2}\rm{GeV}^{-1}$. Both results are in
agreement the Standard Model.
