A test protocol which was developed for on-line dissolved oxygen meters was evaluated in a 60-day field test conducted in an activated sludge aeration basin with thirteen meters from seven cooperating manufacturers. The paper discusses the important features of the test protocol and describes the procedures and results of the field test. These results provide information on methods for testing, calibrating and performance monitoring, information useful for developing specifications, and information on routine maintenance for on-line dissolved oxygen meters in a specific plant environment.