Previous studies on the Darriwilian (Middle Ordovician) Konservat-Lagerstätte of the Winneshiek Shale in Iowa (USA) have reported various animal and trace fossils. A search for “small carbonaceous fossils” (SCFs) in palynological samples from the Winneshiek Shale has now led to the discovery of several different kinds of organic-walled microfossils. Here we report on a particular group of filamentous microfossils that occur abundantly throughout the exposed and subsurface successions of the Winneshiek Shale. The fossils are characterised by large, elongated cells (220–600μm in length, 60–240μm in diameter) with thin and delicate walls and occasional branching. The cells often contain dark internal bodies, most likely condensed protoplasmic remains. Together, these features identify these fossils as eukaryotic rather than cyanobacterial in origin. More specifically, the cell size, cross-walls and branching pattern are shared with forms of benthic ulvophycean green algae, a group with a long but sporadic fossil record that is otherwise restricted to Proterozoic Lagerstätten. The new specimens therefore expand the known diversity of local primary producers in the palaeoenvironment of the Winneshiek Shale, and suggest that the apparent dearth of delicate filamentous green algae in the Phanerozoic record may be, in part, an artefact of low preservation potential combined with destructive processing techniques.