Abstract— The isolation of a mutant, strain PM‐9, of Rhodopseudonionus spheroides with an abnormal complement of carotenoid pigments is described. PM‐9 accumulates phytoene, phytofluene, ζ‐carotene and neurosporene. Semi‐aerobic cultures form more ζ‐carotene and neurosporene relative to total carotenoids than do photo‐synthetic cultures. PM‐9 is killed on exposure to light and oxygen. By making use of the effect of oxygen on the nature of the carotenoids in PM‐9, we have shown that these pigments do not directly influence the in vivo spectrum of bacteriochlorophyll. Diphenylamine inhibits the synthesis of coloured carotenoids in Rhodopseudonionos gelatinosa but does not change the bacteriochlorophyll spectrum.