  We analyze the cosmological constraints that Archeops places on adiabatic
cold dark matter models with passive power-law initial fluctuations. Because
its angular power spectrum has small bins in l and large l coverage down to
COBE scales, Archeops provides a precise determination of the first acoustic
peak in terms of position at multipole l_peak=220 +- 6, height and width. An
analysis of Archeops data in combination with other CMB datasets constrains the
baryon content of the Universe, Omega(b)h^2 = 0.022 (+0.003,-0.004), compatible
with Big-Bang nucleosynthesis and with a similar accuracy. Using cosmological
priors obtainedfrom recent non-CMB data leads to yet tighter constraints on the
total density, e.g. Omega(tot)=1.00 (+0.03,-0.02) using the HST determination
of the Hubble constant. An excellent absolute calibration consistency is found
between Archeops and other CMB experiments, as well as with the previously
quoted best fit model.The spectral index n is measured to be 1.04 (+0.10,-0.12)
when the optical depth to reionization, tau, is allowed to vary as a free
parameter, and 0.96 (+0.03,-0.04) when tau is fixed to zero, both in good
agreement with inflation.
