A sample of Z0 decays containing b-flavoured hadrons is tagged using leptons, and events having precise proper time measurements are selected. These events are used to study B0s oscillations. The flavour (b or b¯ ) at decay is determined from the lepton charge while the flavour at production is determined from jet charge or the charge of a second lepton, where available. The experiment was not able to resolve the oscillatory behaviour, and we deduce that the B0s oscillation frequency Δms>5.2 ps−1 at the 95% confidence level.