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A co-operative taxonomic study has been performed on slowly growing photochromogenic mycobacteria (Runyon Group I) and closely related organisms. Phenetic data on 54 strains, studied in seven laboratories, were collected and analysed by numerical taxonomic methods. Immunological properties and phage susceptibility patterns were analysed independently to establish correlation with numerical classification. Mycobacterium gastri, M. kansasii and M. marinum appeared as distinct well-defined clusters and the serological and phage data supported the resolution of these three species. A table of definitive properties is presented. Two strains each of M. simiae and M. asiaticum formed a loose cluster which was clearly separated from the previously mentioned three species; the small number of strains examined precluded the establishment of a list of definitive properties of these two species. It is concluded that the Runyon Groups, which provided a practical though arbitrary basis for establishment of a series of co-operative studies, have served their purpose and should now be supplanted by classification and nomenclature based on species.