The tick size regime on systematic internalisers (SIs) was seen as a necessary action to level the playing field between SIs and other trading venues, with the hopes to improve the market composition and market quality in favour of regulated markets. However, the previous literature objects to the view as SIs may have gained the first-mover advantage from their previous tick size exemption. This thesis aims to examine whether the MiFID II tick size regime implementation for SIs on June 26, 2020, alters the market composition and improves the market quality at Nasdaq Stockholm. We consider 45 Swedish stocks with the highest daily average turnover to conduct difference-in-difference regressions. We find that the market quality worsens at Nasdaq Stockholm, while the market composition remains unaffected by the SI tick size regime implementation. The quoted spreads, effective spreads and price impact increase at Nasdaq Stockholm following the SI tick size regime. Impatient traders who trade on information may have rerouted their orders to Nasdaq Stockholm after the event since SIs can no longer offer the avoidance of the price-time priority. Therefore, SIs may have an important role in attracting informed traders who consume rather than supply liquidity. However, the interpretation of the results is conditioned on an inflow of traders from SIs to Nasdaq Stockholm, which we cannot explicitly measure due to the order flow not being adjusted for orders less than the standard market size.