A core drilled at Salles, southwest of Bordeaux (France), yielded a section of strata which embraces the Oligocene-Miocene boundary. The beds are principally limestones with a predominance of carbonates in opposition to small amounts of detrital elements. On the basis of the frequency of the fauna included in the limestones three sedimentary zones are distinguished. The basal zone contains allochemical components suggesting calm sedimentation and includes nummulites and abundant melobesiids. The intermediate zone has faunal and lithologic characteristics of both the underlying and overlying zones. In the upper zone Miogypsinoides fades out and glauconite and pyrite appear. Examination of the foraminifera and ostracod microfaunas reveals three biozones which correspond to the sedimentary zones. Two tables give the distribution of the principal foraminifera and ostracod species. The type of sediments and the distribution of the microfaunas indicate that the basal zone is Oligocene, the upper zone is Miocene and the intermediate zone marks the top of the Oligocene.