A new ant species Lasius (Lasius) neglectus spec. nov. is described from Budapest, Hungary. The workers are morphologically similar to L. alienus, but queens and males are clearly different and are much smaller. The species is polygynous, without apparent colony barriers, there are probably no nuptial flights, and intra-nidal mating has been observed; in this respect L. neglectus has several features in common with the polygynous Japanese species L. sakagamii. Workers forage in huge masses on aphids in tress. It is argued that the rather late discovery of L. neglectus might be due to its arfificial introduction in Budapest from a yet unknown area. The status as a distinct species is further confirmed by allozyme data (Boomsmaet al., 1990, next paper).