About ten years ago Cyperus eragrostis, a species originating in the tropical parts of South America, settled in a trench near the house at Podsabotin 49 in the Goriška Brda. From there the plant spread to similar neighbouring ruderal sites with hydromorphic soil, to roadside ditches and gravelly banks of the Pevmica, a stream on the border between Slovenia and Italy. Applying the standard Central-European phytosociological method we studied the stands in which this species grows together with other hygrophilous and ruderal species. These stands are dominated by character species of the classes Bidentetea tripartitae, Filipendulo-Convolvuletea, Molinio-Arrhenatheretea, Stellarietea mediae and Galio-Urticetea. Cyperus eragrostis is a new adventitious species in the Slovenian flora. Its ephemeral occurrence in Slovenia was first noted in Istria in 2003 (near Portorož, probably in the Sečovlje salt-pans) and later in 2006 (Gračišče). Its presence in Podsabotin and along the Pevmica, however, is not merely transitional (ephemeral); here, this alien species is likely to have already naturalized in the environment with warm and relatively moist sub-Mediterranean climate. Cyperus eragrostis, vrsta, ki je doma v tropskih predelih Južne Amerike, se je pred približno desetimi leti naselila v izkop pri hiši Podsabotin 49 v Goriških Brdih. Od tam se je razširila na podobna okoliška ruderalna rastišča s hidromorfnimi tlemi, v obcestne jarke in na prodnate bregove Pevmice, ki je mejni potok med Slovenijo in Italijo. Po standardni srednjeevropski fitocenološki metodi smo preučili sestoje, v katerih uspeva skupaj z drugimi vlagoljubnimi in ruderalnimi vrstami. V njih prevladujejo značilnice razredov Bidentetea tripartitae, Filipendulo-Convolvuletea, Molinio-Arrhenatheretea, Stellarietea mediae in Galio-Urticetea. Cyperus eragrostis je nova adventivna vrsta v flori Slovenije. Prvič je bilo v Sloveniji zabeleženo njeno efemerno pojavljanje v Istri leta 2003 (pri Portorožu, verjetno v Sečoveljskih solinah) in kasneje še leta 2006 (Gračišče). Njena prisotnost v Podsabotinu in ob Pevmici ni zgolj prehodna (efemerna), temveč se je v okolju s toplim in razmeroma vlažnim submediteranskim podnebjem ta tujerodna vrsta že udomačila (naturalizirala).