  This paper describes a search for the Standard Model Higgs boson using data
from e^+e^- collisions collected at center-of-mass energies of 161, 170 and 172
GeV by the OPAL detector at LEP. The data collected at these energies
correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb^-1, respectively.
The search is sensitive to the main final states from the process in which the
Higgs boson is produced in association with a fermion anti-fermion pair, namely
four jets, two jets with missing energy, and two jets produced together with a
pair of electron, muon or tau leptons. One candidate event is observed, in
agreement with the Standard Model background expectation. In combination with
previous OPAL searches at center-of-mass energies close to the Z^0 resonance
and the revised previous OPAL searches at 161 GeV, we derive a lower limit of
69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence
level.
