Large populations of the phytoflagellates Cryptomonas rostratiformis, C.phaseolus, C.undulata and the phototrophic bacterium Chromatium cf. okenii have been observed in the oxic/anoxic boundary layer of the slightly eutrophic, dimictic lake Schlachtensee during the summer stratification. Vertical distribution of these populations was studied with the help of a new close-interval sampler and by in situ fluorescence measurements with fine spatial (cm to dm) and temporal (h) resolution on four days in 1991, 1994 and 1995. All populations lived close to the chemocline but showed a regular diel vertical migration with daytime ascent and night-time descent and a low migration amplitude. At least for one species - C.rostratiformis - the pattern of migration suggests, that this behaviour has an endogenous origin. As a result of diel vertical migration, the population of C.rostratiformis was spatially separated from the other dominating populations during daytime. Ultimate cause of diel vertical migration was a better light supply of all populations. Another probable advantage was the reduction in grazing pressure, as large percentages of all populations moved into the anoxic, hydrogen sulphide containing water layers during night-time.