Abstract: The invasive garden ant Lasius neglectus Van Loon, Boomsma & ANDRÁSFALVY, 1990 was found and described from Budapest, Hungary. Since then, 16 colonies of the species were located in the area. More localities could probably be detected by systematic screening throughout the country. The majority of the colonies can be found in Budapest and only a few sites are known outside the capital city (in the Botanical Garden of Debrecen and in Érd and Tahi; the latter two are situated close to Budapest). It is known that several arthropod groups have representatives of a myrmecophilous character that are living in ant nests on a voluntary or obligatory basis (e.g., beetles, springtails, mites, crickets, and woodlice). No such phenomenon was published in relation to invertebrates accompanying L. neglectus before. In 2001, we detected the first commensalist isopod in a colony in Budatétény. We identified the crustacean arthropod as Platyarthrus schoblii BUDDE-LUND, 1885 which is a new species in Hungary. It was known only in the Mediterranean region before. After a systematic search by hand sorting, we were able to prove the presence of this isopod species in 60 % of the studied L. neglectus populations. The other well-established representative of the myrmecophilous genus Platyarthrus in Hungary is P. hoffmannseggii Brandt, 1833, which was not found to co-occur with L. neglectus. Similarly, P. schoblii has not yet been found with other ant species. Nevertheless we presume cross-adoption in each case as it happened, e.g., in Spain between the native P. schoblii and the introduced and invasive ant Linepithema humile (Mayr, 1868). The repeated co-occurrence of P. schoblii and L. neglectus could underline the hypothesis concerning the anthropogenic distribution of both species by potted ornamental plants.