A bacterium of the Chromatiaceae family was isolated from a dense layer of phototrophic bacteria at the mid-depth of Lake Kaiike. The bacterium could be purely cultured in the medium given by PFENNIG and LIPPERT (1966). When sulfide was amply present in the medium, a very elongated form of the bacterium, which contained sulfur globules inside the cell, predominated. With decreasing sulfide concentration, it changed to a sulfur-containing and finally to a motile sulfur-free rod-shaped bacterium measuring 1-2 μm in length to achieve the stationary phase of the culture. Some environmental factors, pH, light, temperature, NaCl, vitamin B12 and sulfur compounds, on the bacterial growth were examined.