Pseudomonas aeruginosa secretes a fluorescent siderophore, pyoverdine, when grown under iron-deficient conditions. Pyoverdine consists of a chromophoric group bound to a partly cyclic octapeptide. As a step toward understanding the molecular events involved in pyoverdine synthesis, we have sequenced a gene, pvdD, required for this process. The gene encodes a 2,448- residue protein, PvdD, which has a predicted molecular mass of 273,061 Da and contains two highly similar domains of about 1,000 amino acids each. The protein is similar to peptide synthetases from a range of bacterial and fungal species, indicating that synthesis of the peptide moiety of pyoverdine proceeds by a nonribosomal mechanism. The pvdD gene is adjacent to a gene, fpvA, which encodes an outer membrane receptor protein required for uptake of ferripyoverdine.