Mantis shrimps are fierce predatory crustaceans in modern day oceans. Fossils of mantis shrimps are still a rarity. They occurred back in the Carboniferous, 300 million years ago, but already with quite a diversity of different forms. Interestingly, all these fossils are comparably large, usually several centimetres body length. From the Mesozoic, numerous mantis shrimp fossils have also been reported, the largest specimens being far in the centimetre range as well, but the smallest specimens being only about 4–5 mm. With this data set, we can reconstruct aspects of the life history for (at least some) Mesozoic mantis shrimps, but not for Palaeozoic forms. We report here a new fossil mantis shrimp specimen from the Carboniferous Bear Gulch limestones, USA. The specimen is quite small, in... its rolled-up position about 10 mm across the laterally preserved body, but 25 mm in estimated total length. This is considerably shorter than any mantis shrimp so far known from the Palaeozoic. Hence, the specimen provides an important data point. Furthermore, it provides some indications where we could possibly find more specimens of comparable or even smaller size: The specimen resembles certain co-occurring fossils, representatives of Cyclida, in some aspects, suggesting that among the material of Cyclida we might find more small-sized mantis shrimps.