A measurement of the fraction of hadronic Z decays in which a gluon splits into a bb ̄ pair, gbb ̄, is presented using data collected by ALEPH from 1992 to 1995 at the Z resonance. The selection is based on four-jet events. Events are selected by means of topological cuts and a lifetime tag. The result is gbb ̄=(2.77+/-0.42(stat)+/-0.57(syst))x10- 3.
