Roseiflexus castenholzii is gram negative and is classified as a green, nonsulfur bacteria. Colonies that were found were red to reddish brown in color. Roseiflexus castenholzii is an unbranched multicellular filamentous organism that does not have a defined length but the diameter is .8 to 1.0 μm. R. castenholzii has no flagella and show signs of gliding motility. An unusual aspect of this organism is that it lacks chlorosomes, which can usually be found in related species, and intracytoplasmic membranes.