Lasius alienus is a small brown to dark brownish ant. The scapes and tibia have no erect hairs.
Until Seifert (1992) split this species, small brown to dark brownish Lasius ants with no erect hairs on the scapes and tibia were recorded as Lasius alienus (Foerster). Early records which could be either Lasius alienus or L. psammophilus Seifert, and which have not been re-examined should be denoted Lasius alienus agg. There is now a third dark Lasius species with no standing hairs on the scapes and tibia recognised in Britain. It is the invasive garden ant Lasius neglectus Van loon et al 1990.
Most records are from southern and central England with a few records from Wales and Scotland. It seems to be widely distributed in Britain but absent from Ireland, although previous confusion with L. psammophilus means that it is currently under-recorded.
There are records from Spain to Kazakhstan and in Scandinavia to 55.3°N.