The selection procedure for a new bacteriophage typing set for Pseudomonas aeruginosa is described. The lytic spectra of 113 internationally available phages derived mostly from previous typing sets, had been grouped by numerical allocating techniques applying the unweighted pair group average cluster analysis to the Jaccard-Sneath similarity matrix and the matrix for the transformed Yule correlation coefficient. This had rendered dendrograms demonstrating the similarity existing between the lytic spectra of the various phages. From each of the clusters arising above a phenon line drawn at a level with approximately 25 clusters, one representative phage was selected. Accordingly, a series of alternative sets were constructed and analysed for their best conformity to the criteria: (a) lysis of the largest possible percentage of bacterial strains, (b) good differentiation, and (c) convenient number of reactions per plate. On this basis a new primary phage typing set for P. aeruginosa of 19 phages and an auxiliary phage typing set of 5 phages were selected.