Data which had been used to characterize the species Pseudomonas fluorescens (Rhodes, 1959), as distinct from the data used to define the genus Pseudomonas, have now been used to obtain an assessment of the over-all similarity of 169 isolates of polarly flagellate bacteria (mainly of the P. fluorescens species-group) by means of an electronic computer. The results of this analysis are compared with the arrangement of isolates previously favoured by the author. It was found that the two arrangements were often closely similar; the discrepancies are discussed. The two aeromonads of the collection were picked out by the computer, which also satisfactorily replaced an accidentally misplaced pseudomonad isolate. One hundred and thirty-four isolates previously considered to belong to the P. fluorescens species-group were positioned together by the computer in a group with more than 80.0% over-all similarity throughout. This confirmed previous conclusions about the validity of this taxonomic unit, even though the description of it may appear imprecise because each character was not possessed by every isolate. The possibility of further species differentiation within this group is discussed, particularly with regard to P. aeruginosa. The results suggest that P. aeruginosa may be regarded as a variety of P. fluorescens. Factors governing the choice of a type strain or culture are discussed.