Personalities seem to be present in most animal taxa and affect many aspects of an individual’s behaviour. Accordingly, animal personality research can provide new knowledge to important ecological questions, such as in the field of invasion biology. Behavioural traits such as boldness and aggression do often correlate within individuals and are thought to be important during the colonisation of invasive species. Further, aggression in invasive species may facilitate the displacement of native species. In this study, I tested the hypothesis that the outcome of interspecific competition between an invasive and a native species can be predicted by the invaders boldness. Round gobies, that are invasive to the Baltic Sea, were assayed for consistency in boldness and then matched against native sand gobies in one-on-one contests over a single shelter. Boldness was consistent within individual round gobies over time. However, boldness had no effect on the outcome of resource competition and there was no correlation between boldness and aggression within individual round gobies. In addition to the main hypothesis, I compared differences in aggression between species and analysed the outcome of contests. My results show that juvenile round gobies and adult male sand gobies compete over hard substrates in lab, that round gobies generally are more aggressive and that the size difference between contestants is the major predictor of contest outcome. These results suggest that sand goby reproduction in the Baltic Sea may be diminished by the invasion and urge for additional field studies.