Résumé : The forward-backward asymmetries of the processes e+e - → Z → bb̄ and e+e- → Z → cc̄ were measured from a sample of hadronic Z decays collected by the DELPHI experiment between 1993 and 1995. Enriched samples of bb̄ and cc̄ events were obtained using lifetime information. The tagging of & and c quarks in these samples was based on the semileptonic decay channels b/c → X + μ and b/c → X + e combined with charge flow information from the hemisphere opposite to the lepton. Combining the AFB bb̄ and AFBcc̄ measurements presented in this paper with published results based on 1991 and 1992 DELPHI data samples, the following pole asymmetries were obtained: AFB 0,b = 0.1021 ± 0.0052 (stat) ± 0.0024 (syst) A FB0,c = 0.0728 ± 0.0086 (stat) ± 0.0063 (syst) The effective value of the weak mixing angle derived from these measurements is sin2 θW,efflept = 0.23170 ± 0.00097.