We study a two-period model in which two classes of dealers intermediate liquidity traders’ orders, and where markets are fragmented due to both an informational and a participation friction. ‘Fast’ dealers continuously monitor the market, thereby trading at both dates; normal dealers, instead, only trade in the first period. This setup generates propagation of liquidity shocks, liquidity complementarities, and potential multiple equilibria with different levels of liquidity. The model can explain market fragility and “flash crashes.” We assess the impact of a decrease in fragmentation on market quality and traders’ welfare. An increase in the mass of fast dealers can decrease liquidity. However, with transparent markets, liquidity and welfare are always increasing with market participation. Furthermore, equilibria with high liquidity are not necessarily associated with higher welfare.

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