Fresh waters rich in nutrients often show mass development of cyanobacteria. The kind of cyanobacteria to be found depends on the properties of the lake. In non-stratified shallow lakes, the most common species Oscillatoria agardhii. In stratified lakes, cyanobacteria can be found in restricted zones of the deeper part of the lakes, and always possess cells with very active gas vacuoles. The most common species are Microcystis aeruginosa, Oscillatoria agardhii var isothrix, Oscillatoria "var red" and different Anabaena species. In lakes with prolonged nitrogen limitation, genera such as Anabaena and Aphanizomenon are common. Two important factors determine the distribution of these organisms: the light climate and the availability of nutrients. We have limited our discussion in this paper to the influence of the light climate. This influence of the light climate on growth can be determined in different ways. The influence of light intensity on the growth rate of Oscillatoria agardhii can be described using a Monod-like relation with light inhibition at light intensities above 40 W m-2. Studies with other species of cyanobacteria like Microcystis aeruginosa and Aphanizomenon flos-aquae support the assumption that most cyanobacteria living in fresh water are extremely sensitive to high light intensities. The energy balance of phototrophic growth can be described by the equation mu = qE . c - mue where mu is the specific growth rate, mue the specific maintenance energy rate and qE the specific light energy uptake rate; c represents the growth efficiency factor with which light energy is converted into cell material. In this equation, there are two factors which determine the growth of the organisms: ue and c. It was found that the value of mue of Oscillatoria was extremely low (mue approximately 0.001 h-1) compared with a eukaryotic organism (Scenedesmus: mue 0.008 h-1). The value of c was found to be dependent on the growth rate, but did not greatly differ from values found in eukaryotic algal cells. Data obtained from cultures of Oscillatoria grown under light-dark cycles, so that total energy input over 24 h was growth-limiting, showed good regulation of both the carbohydrate synthesis rate and the growth rate. In comparing the data of light-limited growth of cyanobacteria, the conclusion can be drawn that these organisms are favoured at low light intensities by their low specific maintenance energy rate and their pigment composition. A number of competition experiments support the conclusion that the light climate is the most important steering factor for the distribution of cyanobacteria in fresh waters.