The branching fraction of Z --> bb relative to all hadronic decays of the Z has been measured using event shape variables to preferentially select Z --> bb events. The method chosen applies a combination of shape discriminators and the selection of high transverse momentum leptons to event hemispheres. From a sample of 440 000 hadronic Z decays collected with the ALEPH detector at LEP, the ration Γbb/Γhad = 0.228+/-0.005(stat.)+/-0.005(syst.) is measured.
