In 431 000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions ofbbar b andcbar c events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at the Z mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR(b to ell ^ - bar vX) and BR ( b→ cl+ vX) yield 0.114±0.003±0.004 and 0.082±0.003±0.012, respectively. The dilepton events enable measurement of the b mixing parameter, Χ=0.114±0.014±0.008. Results are also presented for the energy variation of thebbar b asymmetry and the parameters required to describe heavy quark fragmentation. From the asymmetry measurements, the effective electroweak mixing angle is sin2θ{/W eff}=0.2333±0.0022.
