There is hardly any group of unicellular organisms that has attracted the interest of so varied a group of biologists as the purple bacteria. Their shape, their size, their color, their peculiar reactions, chemotactic as well as phototactic, their mass-occurrence under specific conditions, all have furnished material for the taxonomist, the physiologist, the ecologist and the biochemist. Not less than five monographic papers on this group of organisms have appeared ’), besides an overwhelming number of shorter articles dealing with the various aspects of the biology of these bacteria. On perusal of this extensive literature one becomes struck by the peculiar controversies that have existed and for the greater part still do exist concerning the ideas on this group; and this with respect to morphological as well as taxonomic and biochemical problems.