The diatom flora of three differing types of oligotrophy lakes was investigated. These lake-types are becoming rarer, even outside Europe. Generally, oligotraphentic species are much less well known than the mostly cosmopolitan diatoms in high densities in eutrophic waters. The numerous ecological investigations over the last decades have given us a good insight into the regional diatom floras of the temperate zones of the northern hemisphere. This is especially true of central, northern and western Europe, North America, Japan, and to a certain extent of South Africa and a few other regions. Recent interest has increasingly concentrated on diatoms from waters effected by man, e.g anthropogenic eutrophication, contamination by sewage, salt influx, acidification by acidic, inorganic emissions. Today, floras in such waters are comprised of a selection of particular, progressively more conimonly occurring diatoms.