SUMMARY: The bacteria of the upper reaches of a chalk stream (Hobson's Brook) were studied as part of an inquiry into their association with ciliate protozoa. The bacteria studied were primarily those found in the current core at 2-month intervals as revealed by plating at 22°. The waters of the stream flow from chalk springs through arable land and are swift, cold, alkaline and usually rich in dissolved oxygen. These physical characters generally restricted the microfiora to psychrophilic aerobes; but once, when turbulence was lost by damming of the water by a growth of Nasturtium officinale, micro-aerophilic and anaerobic bacteria tended to predominate. Near plants, especially blue-green algae, bacterial numbers were always much higher than in the current core. The natural indigenous water flora was supplanted, after floods, continued heavy rain, or periods of drought, by a foreign microflora apparently coming from the adjacent soil. The latter flora was apparently modified by agricultural operations on the surrounding arable land and was partly recruited from the rhizospheres of cereal crops thereon. A tributary stream contaminated by farmyard and domestic waste opens into the Brook but true Escherichia coli were only isolated from its mouth. It is suggested that the topography and turbulence of the Brook combine with protozoan activity to remove polluting bacteria rapidly. Two strains of Gram-positive micrococci isolated from the Brook induced fission in ciliate protozoa. An unusual type of nitrogen-fixing bacterium was isolated on several occasions.