Pseudomonas aeruginosa is an opportunistic pathogen causing severe infections, especially in lungs of patients with cystic fibrosis. Environmental conditions induce the production by the bacteria of a viscous mucoid exopolysaccharide, called alginate, which is one of the most important factor of virulence of P. aeruginosa. Alginate is a linear polymer of beta-1, 4-linked L-guluronic acid and D-mannuronic acid. The alginate biosynthetic pathway involves genes called alg which are clustered at the 34 min region of chromosomal DNA of P. aeruginosa. The key enzyme of alginate biosynthesis, the GDP-mannose dehydrogenase is encoded by the gene algD. Expression of algD is positively controlled by several proteins, especially AlgU, a putative sigma factor homologous to sigma E of E. coli, AlgR and AlgP, a transactivator and an histone-like respectively. Here, a scheme of alginate biosynthetic pathway and a model for the alg genes regulation are described from results published in literature and from our own interpretation.