The Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of northeast Iowa is preserved in a meteorite crater. Besides conodonts, the fossils are dominated by arthropods, particularly eurypterids and phyllocarids. Here we describe the bivalved forms, which include at least seven different taxa. The small phyllocarid Ceratiocaris winneshiekensis, new species, is the most abundant; it is the oldest representative of the Ceratiocarididae. A single incomplete abdomen and telson bearing furcal rami is reminiscent of notostracan branchiopods but its affinities are unknown. Decoracaris hildebrandi, new genus and species, is a rare form with a shield that extends anteriorly into a swollen horn and reaches lengths of 9 cm: it may represent a thylacocephalan crustacean but this cannot be confirmed without soft parts. Iosuperstes collisionis, new genus and species, is represented by suboval valves 10–25 mm long: its affinities are unknown. A probable leperditicopid, which ranges in length from 8 to 18 mm, is commonly preserved in a ‘butterflied’ configuration. It does not preserve the scars and sinuses characteristic of three-dimensionally preserved leperditicopids from elsewhere. Finally the fauna includes at least three ostracods, including a palaeocope with a granular surface and relief similar to Lomatopisthia, and a smooth ?podocope. The Winneshiek fauna differs from those of other Ordovician Lagerstätten from restricted settings such as Airport Cove and William Lake in Manitoba (Katian) where, apart from ostracods, bivalved arthropods are absent, and Silurian examples such as Brandon Bridge (Telychian), which lacks eurypterids, and the Williamsville Member of the Bertie Formation (Pridoli) where conodont assemblages are absent.