AUXOTROPHIC mutants of a strain of Pseudomonos aeruginosa were isolated by Loutit1 following X-ray treatment and using a modification of the penicillin-isolation technique devised by Davis2 and Lederberg and Zinder3. An attempt was made to repeat this work using a new strain of Pseudomonas aeruginosa; but from a series of ten experiments only one auxotrophic mutant was isolated. A further thirty experiments were carried out on this new strain, using ultra-violet irradiation instead of X-rays and employing the same isolation technique. From these thirty experiments only eleven mutants were isolated, representing less than 0.2 per cent of the total colonies examined. Six of the eleven mutants were isolated in one experiment; but five of these had the same nutritional requirements.