Study of prompt $J/\psi$ production in hadronic Z decays Rousseau D CERN 1997 A measurement of prompt J/psi production with low model dependence is performed on 3.9 million hadronic Z decays collected by the ALEPH detector. The sample of J/psi at short decay-length shows an excess 46+/-12 prompt J/psi on a background of 57 B decay and 18 combinatorial events, yielding the branching ratio Br(Z->prompt J/psi X)= (3.00+/- 0.78(stat) +/- 0.30(syst) +/- 0.15(theo)) *10^-4. This result is consistent with a theoretical prediction allowing for color-octet gluon->J/psi as well as c quark fragmentation. PREPRINT