Mutant strains [7] of M. purpureus, induced by fast-neutron and X-ray irradiations, differed in morphology, photoresponses and anti-bacterial activities. Instead of producing mainly ascospores on malt extract agar, most of the new strains produced a tremendous amount of conidia. Slightly pigmented strains had sturdier hyphae with less branching than those highly pigmented strains. Filaments of isodiametric cells occurred in one strain. Pigmentation of albino and slightly pigmented strains was greatly stimulated by UV irradiation. Conidiation of the albino strain was also stimulated. Unknown compound(s) from Bacillus sp. could substitute for the stimulatory effect of UV light. The yellow pigment of the strains is proposed to be the photoreceptor.