Lasius platythorax is a small brown to dark brownish black ant which until recently was confused with L. niger (L). Seifert (1991) showed that it was a distinct species based on morphological differences coupled with distinctly different habitat preferences and and this is now accepted by most authorities. The scapes and tibia have erect hairs. The clypeus has sparse pubescence and the hairs on the pronotum are longer than those of L. niger.
It seems to be widely distributed in Britain and Ireland, but previous confusion with L. niger means that it is under recorded at this time.
Records are restricted to Europe and range from Norway to Corsican mountains.