MqsR-controlled colanic acid and biofilm regulator (McbR, also known as YncC) is the protein product of a highly induced gene in early Escherichia coli biofilm development and has been regarded as an attractive target for blocking biofilm formation. This protein acts as a repressor for genes involved in exopolysaccharide production and an activator for genes involved in stress response. To better understand the role of McbR in governing the switch from exponential growth to the biofilm state, we determined the crystal structure of McbR to 2.1 angstrom. The structure reveals McbR to be a member of the FadR C-terminal domain (FCD) family of the GntR superfamily of transcriptional regulators (this family was named after the first identified member, GntR, a transcriptional repressor of the gluconate operon of Bacillus subtilis). Previous to this study, only six of the predicted 2800 members of this family had been structurally characterized. Here, we identify the residues that constitute the McbR effector and DNA binding sites. In addition, comparison of McbR with other members of the FCD domain family shows that this family of proteins adopts highly distinct oligomerization interfaces, which has implications for DNA binding and regulation.