ABSTRACT. Allelopathy, the release of phytotoxins by plants, may be a key mechanism by which an invasive species displaces residents. However, methodological procedures are still questioned in the literature. We evaluated the phytotoxic effects of Cynodon dactylon and Eragrostis plana, the most invasive grasses of the Río de la Plata grasslands. Although allelopathy has been claimed as one of the possible mechanisms involved in the establishment and impact of these species, no empirical evidence has convincingly supported it. We performed a germination experiment to assess the effect of both invasive species on Eragrostis bahiensis, a native grass from the region. We used aqueous extracts from fresh and dry leaves of three donor species: two invasive (C. dactylon and E. plana) and one co-occurring native from the introduced range (Coelorachis selloana). This approach allows to relativize the invasive species effect from resident species, according to the novel weapon hypothesis. As a result, only aqueous extracts from dried leaves inhibited the germination and early growth of E. bahiensis. The magnitude of these effects varied between donor species, but there was no consensus to conclude both invasive species had greater effects. Cynodon dactylon caused the strongest inhibitory effect on E. bahiensis. However, the native C. selloana also presented a potent inhibitory effect, stronger than the well-known invasive E. plana. Thus, the role of allelopathy on E. plana invasion in the Río de la Plata grasslands should be questioned. Finally, we addressed some suggestions for improving experimental design for testing the novelty of phytotoxic effects in the introduced range.