The highest number of mithraea in urban context of the ancient world come from Ostia. Although we do not know the whole city, mithraea have been found in all districts of the town. The spread and fortune of the Mithraic worship are also attested by the plenteous epigraphic and sculptural materials. This research deals with the Mithraism at Ostia, focusing on the particular case of monograms, just mentioned by Giovanni Becatti in his seminal work about mithraea at Ostia, dating back to more than sixty years ago. After the recent discovery of the Mithraeum of colored marbles by the archaeologists of the Ostia Marina Project (University of Bologna), it seems necessary to examine and contextualize the phenomenology of Mithraic monograms at Ostia, as is done in relation to similar processes which involve the Christian world.