Two chemotactic transducer genes (termed pctB and pctC) and an open reading frame (orf1) were found in the pctA-flanking region which was previously identified as a chemotactic transducer gene in Pseudomonas aeruginosa. The pctB and pctC genes encode predicted polypeptides of 629 and 632 amino acids, respectively. Overall, PctB and PctC had 81 and 75% amino acid identities with PctA, respectively. A null mutant strain PCT2, which contained a deletion in the entire pctC, orf1, pctA and pctB genes, did not show chemotaxis towards all 20 commonly occurring L-amino acids. This mutant strain also failed to respond to amino acid catabolites (cadaverine, 4-aminobutyrate and putrescine) that are strong attractants for the wild-type strain PAO1. To study the role of each gene product in L-amino acid taxis, plasmids harbouring the pctC, orf1, pctA, or pctB genes were constructed and introduced into strain PCT2 by transformation. The orf1 gene did not complement the defect in chemotaxis of strain PCT2. The pctA gene restored the ability of strain PCT2 to respond to 18 L-amino acids, suggesting that PctA plays a major role in detecting L-amino acids in P. aeruginosa. The pctB and pctC genes complemented the defect in chemotaxis to only seven (Ala, Arg, Glu, Lys, Met, Tyr, Gln) and two (His, Pro) L-amino acids, respectively.