The hadronization of quarks into jets of hadrons can currently only be
described phenomenologically. A large number of models and
parametrisations exist, and lead to different predictions for the
energy distribution of the hadrons created in this process. Precise
measurements of this distribution, especially for hadrons containing
heavy quarks, contribute to a better understanding of the
hadronisation process. Furthermore, they are an important ingredient
for many other heavy quark precision measurements, where limited
knowledge of the energy distribution often leads to large systematic
uncertainties.

A measurement of the energy distribution of b hadrons in about 4
million Z decays recorded in 1992--2000 with the OPAL detector at LEP
is presented. The statistical uncertainty is minimized by inclusive
reconstruction of b hadrons. The average energy of the b hadrons is
measured model-independently. In addition, common hadronisation models
and parametrisations are compared to OPAL data.