An organism phenetically indistinguishable from was isolated from (ATCC 14470) by successive ultraviolet irradiations after three selection steps. From the parent (a smooth-colony-forming, scotochromogenic strain) were isolated: (i) photochromogenic mutants; (ii) rough-colony-forming mutants; and (iii) non-photochromogenic (pigmentless) mutants. In the third step of mutation, some mutants showed simultaneous changes of other characters, e.g., loss of the ability to utilize several carbon sources. After ultraviolet irradiation, mutant colonies comprised 1 to 2 per 10 surviving colonies. It was impossible to isolate non-photochromogenic mutants directly from parent scotochromogenic bacteria. Mutants could be isolated only from photochromogenic mutants. This is in marked contrast to a previous work in which non-photochromogenic mutants were isolated directly from parent, scotochromogenic .


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