Ostracodes occur in shallow marine limestones and deeper marine graptolite-bearing mudstones of the Ordovician (Caradoc Series) Craighead Limestone Formation, SW Scotland. The fauna of the limestones, a well preserved, palaeocope-rich association, comprises Oepikella (O. tunnicliffi sp. nov.), Platybolbina, Levisulculus, Distobolbina and Leperditella species, and rare podocopes including Monoceratella teres Teichert 1937a. These taxa were previously unknown from the British Ordovician. The faunas of the mudstones are mostly indeterminate moulds. Monoceratella teres has a widespread occurrence in Ordovician carbonate-rich sequences of the eastern United States, but is stratigraphically long-ranging (North American late Whiterockian – early Mohawkian series). The North American affinity of the fauna from the limestone is also indicated by Platybolbina cf. punctata (Kraft 1962), Levisulculus cf. michiganensis Kesling 1960a and Leperditella sp., although at generic level, most of the ostracode taxa also occur in Scandinavia. This supports the suggestion that, by Caradoc times, the Iapetus Ocean no longer represented a barrier to migration between shallow marine-shelf ostracodes of the Laurentia and Baltica palaeocontinents.