The growth of two phytoplanktons was studied in a natural environment and in the laboratory under artificial radiation conditions in the presence of absence of UV radiation. The effect of an elevated temperature on the two phytoplanktons was also examined. UV radiation resulted in a decrease in the growth of the two phytoplanktons; P. micans was more affected by UV than S. costatum. Four hours of UV radiation decreased the motility of S. costatum and P. micans by 20 % and 40 %, respectively. Accordingly, an elevated temperature and UV radiation decreased the growth rate of the two phytoplanktons investigated.