  The mass and width of the W boson are determined in e+e- collisions at LEP
using 183 pb^-1 of data recorded at a centre-of-mass energy roots=189 GeV with
the OPAL detector. The invariant mass distributions from 970 WW->qqqq and 1118
WW->qqln candidate events are used to measure the mass of the W boson,
  Mw = 80.451 +- 0.076(stat.) +- 0.049(syst.) GeV.
  A direct measurement of the width of the W boson gives Gw=2.09 +- 0.18(stat.)
+- 0.09(syst.) GeV. The results are combined with previous OPAL results from 78
pb^-1 of data recorded with roots from 161 to 183 GeV, to obtain:
  Mw = 80.432 +- 0.066(stat.) +- 0.045(syst.) GeV,
  Gw = 2.04 +- 0.16(stat.) +- 0.09(syst.) GeV.
  The consistency of the direct measurement of Mw with that inferred from other
measurements of electroweak parameters provides an important test of the
Standard Model of electroweak interactions.
