A Thiococcus sp. nov. gen., the first Thiorhodaceae known to contain bacteriochlorophyll b, was isolated from river mud using filtered light above 900 mμ as the selecting agent in enrichments. The orange-brown colour of the immotile, Gram-negative, photolithotrophic cocci is due to two new carotenoids, 3,4,3′,4′-tetrahydrospirilloxanthin and 3,4,3′,4′-tetrahydrospirilloxanthinal. The photosynthetic pigments appear to be associated with a unique internal membrane system consisting of branched membrane tubes of even diameter (approx. 450 A) continuous with the cell membrane. The negatively stained tubes examined in the electron microscope reveal a surface fine structure of raised bodies arranged in a regular hexagonal pattern.