Bacterial motility and chemotaxis are typically viewed as single-cell behaviors, used by individual bacteria to identify conditions that are favorable for growth. For the well-studied model Escherichia coli, we investigated the roles of motility and chemotaxis in multicellular behaviors of bacteria, including formation of multicellular aggregates and surface-attached biofilms as well as high-density swarming. We show that chemotactic self-attraction mediated by a quorum-sensing molecule AI-2 can largely enhance autoaggregation and biofilm formation by E. coli, and foster formation of aggregates in mixed bacterial communities.  Furthermore, we demonstrate that collective motion that emerges at very high densities of motile bacteria suppresses chemotaxis, interfering with the mechanism of gradient sensing by bacteria.