The late Katian, Late Ordovician Boda Limestone of Dalarna, Sweden contains a rich cephalopod assemblage. The assemblage consists of 61 species, of 31 genera, comprising almost all major Ordovician cephalopod orders. Most common and diverse are the Orthocerida. The Ascocerida are also remarkably comman and diverse. The new ascocerid species, Redpathoceras bullatum sp. nov., R. depressum sp. nov., R. magnum sp. nov., and Probillingsites scandinavicum sp. nov., give reason to revise current hypotheses on the origin an evolution of this group. An ascocerid origin from barrandeoceratids or aspidoceratids is hypothesised. The absence of actinocerids in the Boda Limestone is notable, and is interpreted as an indication of relatively cool or/and deep depositional environments. The dominance of orthocerids is provisionally interpreted as evidence for nutrient-rich waters during the time of the deposition of the Boda Limestone. Additionally, the assemblage contains the new barrandeocerids Schuchertoceras fryi sp. nov., Siljanoceras varians gen. et sp. nov., Warburgoceras gen. nov. (for Cyrtoceras longitudinale Angelin in Angelin & Lindström, 1880), the new endocerid Cameroceras turrisoides sp. nov., the new oncocerid Cyrtorizoceras thorslundi sp. nov., and the new orthocerids Dawsonoceras stumburi sp. nov., Isorthoceras angelini sp. nov., I. curvilineatum sp. nov., Nathorstoceras adnatum gen. et sp. nov., N. kallholnense gen. et sp. nov., Palaeodawsonocerina? nicolletoides sp. nov., Pleurorthoceras osmundsbergense sp. nov., and Striatocycloceras isbergi sp. nov.