A most-probable-number (MPN) technique was evaluated for detecting and enumerating Pseudomonas aeruginosa in water and wastewater. Both the presumptive and confirmatory media, as described in the 13th edition of Standard Methods for the Examination of Water and Wastewater, as well as modifications of these media were included in evaluations. Various samples of water were tested, namely chlorinated tap water, creek water, and influent to a wastewater treatment plant. Modified media repeatedly gave higher estimated MPNs of P. aeruginosa than media listed in Standard Methods. P. aeruginosa was detected and recovered from all creek water and wastewater samples, but not from tap water samples tested. This organism was determined to be present in as large numbers as the fecal coliforms and in even greater quantities than the fecal streptococci in all samples, whenever MPN estimations were determined from those positive tubes containing the modified confirmatory medium.