SummaryThePseudomonas aeruginosaexopolysaccharide alginate is an important virulence factor in chronic pulmonary infections of cystic fibrosis patients. We determined the nucleotide sequence of the gene,algB, which regulates the level of exopolysaccharide produced by mucoidP. aeruginosa.The predicted amino acid sequence of AlgB revealed a high degree of similarity to the regulatory proteins in the NtrC subclass of ‘two‐component regulatory systems’. AlgB expression inEscherichia coliminicells showed a molecular weight of ˜ 50 000 Da, comparable to that of the inferred amino acid sequence (49 318 Da). We show thatalgBis transcriptionally active in mucoid strains ofP. aeruginosaand regulates the expression of the alginate biosynthetic gene,algD, thereby resulting in increased expression of alginate in mucoidP. aeru