The chrysophyte Synura sphagnicola Korsch. was isolated from a hypolimnion bloom in a Canadian Shield lake and its ability to grow at low light and temperature was studied. Growth saturating light was much higher than in situ intensities and independent of temperature while compensation intensity decreased with decreasing temperature. Optimum temperature decreased with decreasing light intensity. While optimum temperature was lower than generally seen among temperate water algae, compensating and saturating light were similar to those seen in other algae. At low light and temperature, the growth rate of S. sphagnicola was lower than the growth rates under similar conditions of other algae, and appeared insufficient to account for the net rate of chlorophyll accumulation observed in the bloom from which the alga was isolated.