The host ranges of 113 typing bacteriophages have been studied at their Routine Test Dilution (RTD) for 486 strains ofPseudomonas aeruginosa. The resulting data matrix has been analysed by numerical allocating procedures employed in numerical taxonomy. The Jaccard‐Sneath similarity index (SJS) and the simple matching coefficient (SSM) were calculated and the results of the weighted and the unweighted pair group average cluster analysis (WPGA and UWPGA) compared. The SSMseemed to produce misleading results. Phages that lysed less than 15 per cent of the bacterial strains tended to be grouped together at high similarity levels, regardless of real differences in their host ranges. For such phages, there was a linear relationship between per cent lysis and SSM. The best representation of the similarity index was obtained with the UWPGA applied to the SJS‐matrix ([SJS] UWPGA). This rendered a cophenetic correlation coefficient of 0.9303 in contrast to r = 0.8933 with the [SJS] W