A new medium is described for the isolation of P. aeruginosa organisms which have been damaged by chlorination. This medium utilises the fact that with arginine. P. aeruginosa produces a strongly alkaline reaction resulting in an easily identified colour change from gray-green to blue-violet. The indicators used for this purpose are bromothymol blue and cresol red. The medium may be used with either the membrane filter or the liquid enrichment technique. Incubation is at 37 degrees C for 24 or 48 h.