The pgaABCD operon of Escherichia coli is required for production of the biofilm adhesin poly-beta-1,6-N-acetyl-d-glucosamine (PGA). We establish here that NhaR, a DNA-binding protein of the LysR family of transcriptional regulators, activates transcription of this operon. Disruption of the nhaR gene decreased biofilm formation without affecting planktonic growth. PGA production was undetectable in an nhaR mutant strain. Expression of a pgaA'-'lacZ translational fusion was induced by NaCl and alkaline pH, but not by CaCl(2) or sucrose, in an nhaR-dependent fashion. Primer extension and quantitative real-time reverse transcription-PCR analyses further revealed that NhaR affects the steady-state level of pga mRNA. A purified recombinant NhaR protein bound specifically and with high affinity within the pgaABCD promoter region; one apparent binding site overlaps the -35 element, and a second site lies immediately upstream of the first. This protein was necessary and sufficient for activation of in vitro transcription from the pgaA promoter. These results define a novel mechanism for regulation of biofilm formation in response to environmental conditions and suggest an expanded role for NhaR in promoting bacterial survival.