The average lifetime of b hadrons has been measured using the L3 detector at LEP, running at √s ≈ M_Z. A b-enriched sample was obtained from 432538 hadronic Z events collected in 1990 and 1991 by tagging electrons and muons from semileptonic b hadron decays. From maximum likelihood fits to the electron and muon impact parameter distributions, the average b hadron lifetime was measured to be τ_b = (1535 ± 35 ± 28) fs, where the first error is statistical and the second includes both the experimental and the theoretical systematic uncertainties.Facultad de Ciencias Exactas