THE growth of nitrifying bacteria (Nitrosomonas and Nitrobacter) is usually inhibited by comparatively low concentrations of soluble organic substances, particularly amino-compounds1–3. A rather exceptional resistance was shown by a strain of Nitrosomonas that was isolated from farmyard manure and in other respects appeared typical (oval rods, motile by means of a single polar flagellum, no growth in ordinary bacteriological media, no nitrite formation except from ammonia).