The increase of cholinesterase (ChE), acid phosphatase (Ac.Pase), and phospholipase C (PLC) activities by Pseudomonas aeruginosa was associated with the choline consumption in growth media of varied composition (high or low P-i concentrations, presence or absence of ammonium ion, amino acids, polyamines, peptone, or tricarboxylic acid cycle intermediates). The highest production of the three enzymes occurred in the late stationary growth phase. The simultaneous presence of alkaline phosphatase (Alk.Pase) and the above enzymes was noted when the bacteria were grown in low P-i medium plus choline, in the absence of a preferred carbon source. The importance of choline in the production of ChE, Ac.Pase, and PLC was observed in either clinical isolates or collection strains of P. aeruginosa. These enzymes catalyze the hydrolysis of acetylcholine, phosphorylcholine, and phosphatidylcholine. Through their action the bacteria may break down various compounds (e.g., acetylcholine, from the corneal epithelium; lung surfactant dipalmitoylphosphatidylcholine; phosphorylcholine, a product of the PLC action) or cell membranes through the coordinated action of PLC and Ac.Pase or Alk.Pase. The final consequence of the action of these enzymes is an increase of the free choline concentration. Extrapolated to an in vivo situation, if the stationary growth phase resembles the conditions that P. aeruginosa encounters in its natural environments, then it is possible to include choline among the factors promoting the pathogenicity of this bacterium.