Abstract : A search for B0_s oscillations is performed in a sample of semileptonic B hadron decays collected by the ALEPH experiment during 1991--1995. Compared to previous inclusive analyses, the proper time resolution and mesons flavour tagging are significantly improved. In addition, subsamples of the data are enriched in B0_s. Unbinned maximum likelihood fits are performed to derive: Delta m_s > 9.4 ps-1 at 95% confidence level, which is the highest limit to date. Combining with the ALEPH D-_s based analyses yields Delta m_s > 9.5 ps-1 at 95% confidence level. This provides a significant constraint on the right side of the Unitarity Triangle of the CKM matrix.