Pyritized filamentous cyanobacteria have been discovered by scanning electron microscopy of Ordovician brachiopods from nineteenth century collections in the Natural History Museum, London. The cyanobacteria form mats on strophomenid brachiopods from the Cincinnatian Group (Upper Ordovician, Katian) near or in Cincinnati, Ohio. There is no additional stratigraphic or locality information. The cyanobacteria are found only on the concave exterior surfaces of dorsal valves. Most comprise uniserial, unbranching strands of cells that range from 5 to 9 micrometers in length and width. Some strands are up to 700 micrometers long and most are sinuous. There are no heterocysts or akinetes. The pyritic molds of some cells have a reticulate structure on their surfaces, with each unit a few hundred nanometers in diameter, interpreted as due to framboid microcrystal growth within an organic matrix. A few specimens show what may be extracellular matrix. These Ordovician microbes resemble cyanobacteria belonging to the extant Order Oscillatoriales (Subsection III). The cyanobacterial mats overgrew encrusting cyclostome bryozoans, including broken zooids and terminal diaphragms, sharing the same brachiopod substrates. Since the cyanobacteria are inferred to have been photosynthetic, the encrusted shells probably rested with their dorsal valve exteriors upwards in the photic zone. The mats likely developed shortly before burial of the brachiopod shells. That Cincinnatian brachiopod shells were colonized by cyanobacteria has been previously demonstrated by microborings, but this is the first direct evidence of microbial mats on the shell surfaces. The mats likely played a role in the paleoecology of sclerobiont communities, and they may have influenced preservation of the shell surfaces by the “death mask” effect.