This is the first empirical evidence on the competition between high-frequency traders (HFTs) and its influence on market quality. We exploit the first entries of international HFTs into the Swedish equity market in 2009 and conduct a difference-in-differences analysis using trade-by-trade data. To further identify the effect, we use the Federation of European Securities Exchanges (FESE) tick size harmonization as an exogenous event that caused HFTs to start trading in stocks. When HFTs compete for trades their liquidity consumption increases. As a result, liquidity deteriorates signifcantly and short-term volatility rises.