The results of an investigation on the division of labour in the guest-ant Formicoxenus provancheri, conducted by observing colonies containing individually marked adults, are presented. Five colories of Formicoxenus were installed in the laboratory with their hosts, Myrmica incompleta, in order to document, over 10 d, the location of individuals as well as individual and interactive types of behaviour. The results show that each colony of Formicoxenus consists of three groups: a group of nurses who remain in the Formicoxenus nest (some 21% of the colony's members); a group of scouts who spend most of their time in the external area (18%) and a very large group of individuals specialized in licking (‘shampooing’) the host to obtain regurgitations (61%), who essentially remain in the Myrmica nest. Division of labour in Formicoxenus appears to be a special adaptation to the xenobiotic way of life. The apparent link between social structure and the probability of profiting from trophallactic exchanges with the host species could lead to interesting predictions on the division of labour in other guest-ants.