  We present results of a search for D0-D0bar mixing and
doubly-Cabibbo-suppressed decays of the D0 in Fermilab experiment E791, a
fixed-target charm hadroproduction experiment. We look for evidence of mixing
in the decay chain D* -> pi D0 -> (K pi or K pi pi pi). If the charge of the
pion from the D* decay is the same as the charge of the kaon from the D decay
(a ``wrong-sign'' event), mixing may have occurred. Mixing can be distinguished
from other sources of wrong-sign events (such as doubly-Cabibbo-suppressed
decays) by analyzing the distribution of decay times. We see no evidence of
mixing. Allowing for possible CP violation, our results for r_{mix}, the ratio
of mixed to unmixed decays, are r_{mix}(D0bar -> D0)=(0.18_{-0.39}^{+0.43} +-
0.17)% and r_{mix}(D0 -> D0bar)=(0.70_{-0.53}^{+0.58} +- 0.18)%. The
sensitivity of these results is comparable to that of previous measurements,
but the assumptions made in fitting the data are notably more general. We
present results from many fits to our data under various assumptions. If we
assume r_{mix} = 0, we find that the ratios of doubly Cabibbo-suppressed decays
to Cabibbo-favored decays are r_{dcs}(K pi) =(0.68_{-0.33}^{+0.34} +- 0.07)%
and r_{dcs}(K pi pi pi)=(0.25_{-0.34}^{+0.36} +- 0.03)%.
