Uruguayan mussel beds are located along the estuarine gradient caused by the interaction of the Río de la Plata run-off and the Atlantic Ocean, changing in species composition across this gradient. In the oceanic portion of the gradient, the exotic Perna perna showed a sequence of local extinction–colonization episodes since at least the second half of the 20th Century, thus altering the dominance relationships within the mussel beds. Owing to its larger size, this species may have caused profound effects on the spatial structure of the mussel beds and consequently on its associated macrofauna. In this vein, we analysed patterns in zonation, abundance and dominance of mussel species in a mussel bed located in the oceanic portion of the Uruguayan coast in relation to tidal and exposure level. We also evaluated if these factors could affect the dominance patterns between mussel species. The main results showed that the dominance of the native mussel Brachidontes rodriguezii was consistent across all tidal levels in all sites in terms of absolute and relative abundance, with the exception of the shallow subtidal at the intermediate site, where P. perna dominated over this species in terms of relative abundance. This suggests that the effect of an invasive mussel is highly dependent on the receptive assemblage, and that the outcome of interspecific competition can also be modulated by small-scale factors.