The lithostratigraphy and ammonite biostratigraphy of the Middle Jurassic Opalinuston Formation in the Teufelsloch gorge at Bad Boll (middle Swabian Alb), which is the type locality of this formation and that of its lower member, the Teufelsloch Member, is described. The term Comptum Subzone, formerly well established in SW Germany, is replaced by the Bifidatum Subzone. Additionally, the regional term Comptumbank bed is replaced by Wilflingen-Bank and the local term Costosumbank bed is replaced by Bifidatumbank. Four ammonite biohorizons (opalinum, dilucidum, opaliniforme and hansrieberi biohorizons) of the Opalinum Subzone and three ammonite biohorizons (bifidatum/rieberi, uncinatum and evolutum biohorizons) of the Bifidatum Subzone (formerly “Comptum Subzone”) are described from the type locality. The crassicostatum biohorizon of the Bifidatum Subzone is introduced in the western Swabian Alb. The biostratigraphical boundary between the Opalinum and Murchisonae zones (Lower/Upper Aalenian) is located at the base of the Achdorf Formation. The succession is compared and correlated with other localities in Germany and Europe. The species of the ammonite genus Leioceras are described using a chronospecies concept. Lectotypes are designated for Leioceras partitum (Buckman, 1899) and Leioceras uncinatum (Buckman, 1899). We briefly report on the macrofauna of the Opalinuston Formation. In addition, we provide important petrographical, sedimentological and geochemical information for the understanding of the depositional conditions for some selected layers.