A comparison was made between the following three methods of classifying bacteria: (1) division on associated characters, (2) identification by the use of Skerman''s key, (3) classification by means of Affinity Index. Division on associated characters was of no value in this particular case. Skerman''s key was adequate for the identification of the isolates, while the Affinity Index (a slight modification of the Similarity Index of Sneath) gave a comprehensive view of the relationships between isolates. It was considered that a random sample of 43 isolates selected from a collection of 318 rhizosphere bacteria formed a spectrum, rather than a series of groups.