How does competition among high-frequency traders affect market liquidity? When high-frequency trading firms compete, does stock market liquidity deteriorate? I argue that the answer is yes. High-frequency trading competition may impact stock market liquidity via two channels. First, more competition is accompanied by more high-frequency trading and larger trading volumes, which improve market liquidity. Second, more competition may mean that high-frequency traders adapt their trading strategies and engage in more speculative trades, which harms market liquidity. Since these two channels have the opposite effects on market liquidity, it is important to disentangle the effects of competition from those of a mere increase in the number/volume of high-frequency trading transactions. In the analysis, I aim to do precisely this, by using an exogenous event which changed the intensity of high-frequency competition for some stocks but not for others. I find that otherwise similar stocks subject to more high-frequency trading competition become less liquid.