Lakes are bodies of water differing from each other in depth, in temperature conditions and salinity, in content of organic substances, etc. All this creates an enormous variety of ecological conditions. Problably one of the general characteristics, which so far has not been given enough attention, is the small amount of organic matter dissolved in the water. Although even in the early works of Zobell & Grant (1942,1943) it was shown that aquatic micro-organisms are capable of developing on media where the amount of assimilable dissolved substances does not exceed 10 mg/l, so far isolation and estimation of micro-organisms has been carried out on media containing glucose or peptone at a concentration of 1 to 10 g/l. Thus a large number of micro-organisms has escaped the attention of researchers. The use of organic media of low nutrient content made it possible to throw new light on the character of aquatic microflora. Using nutrient media containing small amounts of organic matter and enriched with deficient growth factors, vitamins etc., it was possible to show that micro-organisms, which do not grow on standard laboratory media but are very numerous in water medium and comprise autochthonous microflora of natural waters, play a major role in the cycle of organic matter. Another important factor which helped in the study of aquatic microflora was a detailed investigation of individual groups of micro-organisms. Much attention had been given to budding and stalked bacteria, gliding bacteria, saprophytic mycoplasmas and so on. The study of photosynthetic bacteria has been particularly advanced, and the group of iron and manganese bacteria was systematized. It was found that distribution of some species of these groups of micro-organisms is closely connected with the conditions of the environment. It was shown that these organisms are often found in extremely narrow ecological niches. A third important factor in recent studies was the use of electron microscopy for the study of the morphology of aquatic bacteria, including those which do not grow on the usual nutrient media. The use of new methods of investigation greatly widened our ideas about the morphological diversity of micro-organisms and their role in the cycle of substances in waters. Thus in this book we have tried to present information on organisms to which little attention had been paid earlier. We have introduced a number of corrections and additions to the description of bacteria included in the latest edition of Bergey's classification key (Buchanan & Gibbons 1974). Many of these corrections are based on our own observations on new species of micro-organisms. All this became possible because the range of our investigations included lakes of different degree of trophic levels, with different hydrological and temperature regimes. We have described the physiology and biochemistry of individual organisms only where it was necessary to elucidate the causes of their development in a given ecological environment.