Five successive, distinct, microfossil assemblages, primarily characterized by their benthic foraminiferal contents, have been recognized in the Mio-Pliocene of the Carboneras-Nijar Basin (SE Spain). The assemblages record a number of fundamental changes in the environment of the area, which have been dated by means of planktic foraminifer biostratigraphy. In the earliest Messinian the central part of the basin was several hundreds of metres deep and had relatively open marine conditions, which were followed by oxygendeficiency, accompanied by slightly deviating salinities and some reduction in waterdepth. Subsequent deposition of Messinian evaporites was interrupted by sparse more open marine sedimentation. During the late Messinian brackish conditions became dominant and the basin filled with delta-plain sediments. The early Pliocene saw a sudden return to open marine, outer neritic conditions, which shoaled to an inner neritic environment in the middle Pliocene. Since the Carboneras-Nijar Basin was close to the connection between the Mediterranean Sea and the Atlantic Ocean, its history shows that Atlantic inflow became more and more severed in the course of the Messinian and was, either completely or virtually, obstructed in its latest part.