Determination of V_ub at the Z-boson resonance

We show that the observation of energetic charged leptons from semileptonic $b$ becays at the LEP and SLC $Z$--boson factories offers a unique opportunity to measure the quark mixing matrix element $V_{ub}$. We present various distributions of the $b$ decay products to show that the $b \to u$ decays can be cleanly separated from the $b \to c$ decays, with a signal of $O(100)$ events per million $Z$ bosons produced.