The fraction of Z(0) --> b (b) over bar events in hadronic Z(0) decays has
been measured by the OPAL experiment using the data collected at LEP betwee
n 1992 and 1995. The Z(0) --> b (b) over bar decays were tagged using displ
aced secondary vertices, and high momentum electrons and muons. Systematic
uncertainties were reduced by measuring the b-tagging efficiency using a do
uble tagging technique. Efficiency correlations between opposite hemisphere
s of an event are small, and are well understood through comparisons betwee
n real and simulated data samples. A value of
R-b= sigma(e(+)e(-) --> b (b) over bar)/sigma(e(+)e(-) --> hadrons) = 0.217
8 +/- 0.0011 +/- 0./0013
was obtained, where the first error is statistical and the second systemati
c. The uncertainty on R-c, the fraction of Z(0) --> c (c) over bar events i
n hadronic Z(0) decays, is not included in the errors. The dependence on R-
c is where
Delta R-b/R-b = -0.056 Delta R-c/R-c,
Delta R-c is the deviation of R-c from the value 0.172 predicted by the Sta
ndard Model. The result for R-b agrees with the value of 0.2155 +/- 0.0003
predicted by the Standard Model.