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A MICRO-ORGANISM identified provisionally as Spirochaeta rosea nom. prov. has been isolated from a marine upper littoral rock-pool at Woods Hole, Mass. The filaments are spiral, 5–200µ long, 0.7µ wide, with a regular pitch of about 7µ and an amplitude of 2µ (Figs. 1 and 2). Transverse cell walls, approximately 2.5µ apart, are revealed by phase-contrast or electron microscopy (Fig. 3). In size and form the organism resembles S. plicatilis Ehrbg. (= Ehrenbergia plicatilis (Ehrbg.) Gieszczykiewicz)1–4, but in mass the cells are seen to be pinkish or brick red. Growth is markedly aerophilic, with an optimum temperature of 35° C. In sea water supplemented with yeast extract (0.5 per cent) and tryptone (0.5 per cent), the doubling time is 100–120 min. At concentrations less than one-half sea water, growth is suppressed.