SummaryCultures ofChlorella pyrenoidosaPringsheim were grown photoautotrophically in the presence of two concentrations (0·25 and 0·50 μm) of atrazine and methabenzthiazuron. The cell density and chlorophyll content were greatly reduced in presence of both herbicides. These herbicides affected net photsynthesis equally without affecting oxygen uptake. However, cell division was more inhibited by atrazine than by methabenzthiazuron, but chlorophyll content per cell was more inhibited by methabenzthiazuron than by atrazi