Epibiontic cells on the surface of the photosynthetic purple sulfur bacterium Chromatium minus, collected several times during the year from 3 different Spanish lakes, were examined using scanning and transmission electron microscopy. The cells attached to the C. minus cell wall by an electron-dense pad, but did not enter the cell. They were ovoidal (about 0.6 Μm wide) when free, and slightly curved rods (0.3×0.6 Μm) when undergoing division. Division only occurred when cells remained attached to Chromatium. A septum was formed, resulting in 2 or 3 curved rods surrounded by a common capsule. Detached daughter cells became ovoidal. The host ultrastructure changed as a result of epibiontic attachment, showing symptoms of cellular degradation. Simultaneously, plaques could be detected on cell lawns formed spontaneously upon cell sedimentation from field samples. © 1983 Springer-Verlag.