Clones of Cryptomonas phaseolus Skuja, Cryptomonas rostratiformis (Skuja) Skuja in Huber-Pestalozi, and Cryptomonas undulata Gervais were isolated from the deep chlorophyll maximum near the oxic/anoxic boundary layer of the mesoeutrophic lake Schlachtensee, Germany. Different autecological features of these species were studied in batch culture experiments. Cryptomonas cf. ovata Ehrenberg and Chroomonas sp. that never dominated in the deep chlorophyll layer were also isolated from Schlachtensee to study their light-dependent growth in comparison to the deep-living species. Cryptomonas undulata, C. cf. ovata, and C. phaseolus had a very low light compensation point (5–7 μmol.m-2.s-1),whereas the growth rate of Chroomonas sp. and C. rostratiformis was positive above 16 and 24 μmol.m-2.s-1. Cryptomonas phaseclus and Chroomonas sp. became photoinhibited above photon flux densities of 92 and 116 μmol.m-2.s-1. Cryptomonas rostratiformis, C. cf. ovata, and C. undulata reached a maximum growth rate at a considerably higher photon flux density (198–250 μmol.m-2.s-1). Cryptomonas phaseolus grew fastest under light-limiting conditions. Chyptomonas phaseolus and C. undulata were best able to suruive prolonged periods of darkness. Cryptomonas phaseolus, C. rostratiformis, and C. undulata did not show any uptake of fluorescent latex beads. When labeled glucose was provided in naturally occurring concentrations, carbon uptake by C. phaseolus, C. rostratiformis, and C. undulata was negligibly small in comparison to cellular carbon content. I suggest that the adaptation to a low-light environment is an important preadaptation for the dominance of C. phaseolus and C. undulata near the freshwater chemocline.