I evaluated (1) the effects of a native submersed aquatic plant species, <i>Vallisneria americana<i>, on the colonization success of a non-native and highly invasive submersed aquatic species, <i>Hydrilla verticillata<i>, through field and greenhouse experiments; and (2) the effects of <i>H. verticillata<i> propagule density on its colonization success of patches dominated by the native species. Results of the field study, located in a tidal freshwater region of Chesapeake Bay, suggest that pre-existing vegetation did not have significant negative effects on <i>H. verticillata<i> colonization. However, pre-existing <i>H. verticillata<i> biomass and <i>H. verticillata<i> colonization success were strongly correlated. In contrast, results from the greenhouse study showed that <i>V. americana<i> had a strong inhibitory effect on <i>H. verticillata<i> colonization by fragments and increasing <i>H. verticillata<i> fragment density again increased colonization success. Reduced water column nutrient concentrations, resulting from <i>V. americana<i> growth, appeared to negatively affect successful rooting and subsequent colonization by <i>H. verticillata<i>.