The net population growth of one Cryptomonas species was studied in an acidified, clearwater Finnish lake at the end of May. The potential growth of the population was also estimated from the pattern and timing of nuclear and cellular division of the alga. The population started to grow when the temperature had risen from 8 to 11°C. Nuclear division in the vernal Cryptomonas population was well phased and happended solely at night. However, the type of division pattern observed, in which mitosis and cytokinesis overlapped, made it difficult to apply the mitotic index technique for measurements of growth rate in situ. These results emphasise the importance of accurate estimation of the duration of mitosis for this method of determining growth rate in situ.