  We measure the time dependence of the ratio of decay rates for D0 -> K+ pi-
to the Cabibbo-favored decay D0 -> K- pi+. The charge conjugate decays are
included. A signal of 3.3 x 10^4 D*+ -> pi+ D0, D0 -> K+ pi- decays is obtained
with D0 proper decay times between 0.75 and 10 mean D0 lifetimes. The data were
recorded with the CDF II detector at the Fermilab Tevatron and correspond to an
integrated luminosity of 9.6 fb-1 for p-pbar collisions at sqrt(s) = 1.96 TeV.
Assuming CP conservation, we search for D0-D0bar mixing and measure the mixing
parameters to be R_D = (3.51 +/- 0.35) x 10^{-3}, y' = (4.3 +/- 4.3) x 10^{-3},
and x'^2 = (0.08 +/- 0.18) x 10^{-3}. We report Bayesian probability intervals
in the x'^2 - y' plane and find that the significance of excluding the
no-mixing hypothesis is equivalent to 6.1 Gaussian standard deviations,
providing the second observation of D0-D0bar mixing from a single experiment.
