  We present the first measurement of the Bs-Bsbar oscillation frequency Delta
m_s. We use 1 fb^-1 of data from p-pbar collisions at sqrt{s}=1.96 TeV
collected with the CDF II detector at the Fermilab Tevatron. The sample
contains signals of 3,600 fully reconstructed hadronic Bs decays and 37,000
partially reconstructed semileptonic Bs decays. We measure the probability as a
function of proper decay time that the Bs decays with the same, or opposite,
flavor as the flavor at production, and we find a signal consistent with
Bs-Bsbar oscillations. The probability that random fluctuations could produce a
comparable signal is 0.2%. Under the hypothesis that the signal is due to
Bs-Bsbar oscillations, we measure Delta m_s = 17.31^{+0.33}_{-0.18} (stat.) +-
0.07 (syst.) ps^-1 and determine |Vtd/Vts| = 0.208^{+0.001}_{-0.002} (exp.)
^{+0.008}_{-0.006} (theo.).
