Estuaries often show clearly recognizable changes in the distribution of organisms along environmental gradients from riverine to fully marine conditions. Surveys performed along the horizontal axis of the Plym and the Yealm Estuaries identified patterns of distribution and abundance of intertidal barnacles and provided a new assessment on the dominance exhibited by the non-native species Elminius modestus in these estuaries. Elminius modestus occurred furthest up in estuaries and was dominant along most of their length, with the exception of few sites closest to the sea; Chthamalus montagui had the most restricted degree of penetration up-estuary; and Semibalanus balanoides occurred at low abundances, with limits of penetration located between those of C. montagui and E. modestus. At many sites, E. modestus was the only barnacle species found. There were changes in the relative abundances of these three species in several particular locations within the Plym and the Yealm in comparison to previous accounts made in the last decades, which, in most cases led to increased dominance of E. modestus. This was mainly due to reductions in the abundances of S. balanoides. Physico-chemical conditions experienced after settlement, especially deposition of silt, exposure and salinity regime contribute to the patterns described here.