In a comparison of two selective media for Ps. aeruginosa containing 0.03% cetrimide, stronger fluorescence was obtained from growth on the medium prepared with King's medium B as the base (CTA 2); although Ps. aeruginosa was not isolated more frequently from burns on this medium than from cetrimide agar made with a Lemco base (CTA 1), results were easier to assess and there were fewer cases of doubtful fluorescence on CTA 2 than on CTA 1. Pyocyanin production on CTA 1 was better than on CTA 2, but for demonstration of pyocyanin production Wahba and Darrell's medium was preferable and showed the pigment in 455/497 (92%) of fluorescent strains of Ps. aeruginosa from burns.A series of tests on 99 strains of Ps. aeruginosa or presumptive Ps. aeruginosa, on 30 strains of other species of Pseudomonas, and on 68 strains of other species of Gram-negative rods showed the value of growth on cetrimide agar and on triphenyl tetrazolium chloride medium, of slime production in gluconate broth, of gas production from nitrate, and of growth at 42 degrees C. as diagnostic aids in the recognition of Ps. aeruginosa.