Particle counting is fast becoming an important tool for water treatmentoptimization and water quality monitoring. As use of this technology grows, sodoes the need for adequate quality control protocols. A simple method for routineverification of on-line particle counter performance is presented. The methodinvolves control charts based on comparisons of on-line and batch particlecounter readings. Discrepancies in these comparisons indicate the need totroubleshoot the particle counters in question. Such control charting has beenimplemented at Southern Nevada Water System of Boulder City, Nevada, and has beenuseful in identifying counting errors in both the raw and finished water on-lineinstruments. The use of this procedure will give early indications of instrumenterrors and will help to avoid unnecessary routine recalibrations. Includes 13 references, figures.