This paper illustrates the results of a survey aimed at deepening available knowledge on the ecology of Ptilostemon greuteri (Asteraceae), a very rare palaeoendemic and relict nano-phanerophyte discovered about 15 years ago on Monte Inici, near Castellammare del Golfo (province of Trapani, north-western Sicily). Two plant communities characterised by P. greuteri are described in detail; they occur in the locus classicus and in a second, recently discovered stand, which is also very localised and threatened by fire. The field investigations revealed that the sites where the species grows are very similar from the ecological point of view; in fact, both of them are located on the steep slopes of deep gullies, benefitting from constantly cool and shady microclimatic conditions, and allowed to a) better identify the currently preferred habitat (ledges, screes at the base of cliffs), b) analyse from a floristic and phytosociological point of view the maquis communities where P. greuteri grows as co-dominant or dominant species, referable to the class Quercetea ilicis, c) to identify the syndynamic role that these coenoses play within the series and microgeoseries of local vegetation. As far as syntaxonomy is concerned, the Malvo olbiae-Ptilostemonetum greuteri ass. nova is described, a basiphilous, thermophilous and shade-tolerant maquis framed in the Oleo-Ceratonion alliance. Moreover, two subassociations are described: 1) typicum subass. nova, corresponding to a pioneer maquis community prevailing on the coarse, loose and mobile debris located along the slopes at the base of rock cliffs, where P. greuteri is clearly dominant and shows a nano-phanerophytic habitus, and 2) euphorbietosum bivonae subass. nova, a primary aspect found of the ledges of carbonate cliffs, where the species can play either a dominant or co-dominant role with other elements of the maquis (Euphorbia bivonae, Chamaerops humilis, etc.), sometimes showing a slightly smaller size.