Optimum growth in uni-algal cultures of Tribonema minus (Wille) Hazen, a dominant alga in blanketing algal mats in the early spring, was obtained at temperatures between 15-25°C and light intensities above 18 p Einstein m“2 s“1. Uni-algal cultures of Spirogyra singularis Nordstedt, dominant in algal mats during summer, grew well between 20-25 °C and the highest light intensities tested (62 p Einstein m 2 s-1). In mixed cultures T. minus outgrows S. singularis below 13 n Einstein m “2 s “1 and 15 °C. At higher light intensities and temperatures levels S. singularis dominates, which agrees with field observations. The growth reduction of T. minus at these higher levels is discussed.