The effects of the herbicide thiobencarb (Saturn) were tested on the growth of phytoplankton community of rice fields as an important parameter in the evaluation of herbicide phytotadcity.. The herbicidal treatments simulated by emergency of rice field and the amounts used of thiobencarb were 1.5 and 1.0 liter feddan. Fourty seven algal taxa were recoded, 10 belonged to cyanophyceae, 19 to dilorophyceae, 15 to bacillariophyceae and 3 to euglenophyeae. The effect of the two thiobencarb treatments differed significantly in their effect on phytoplankarts The high rate was significantly higher in simulating the phytoplankton communities than in the to* rate treatment. The bacillariophyceae forms were relatively resistant to the herbicidal treatments. Thiobencarb was toxic to the growth of some cyanophyceae, Anabaena spimides and Chbrococcus minor. It was lethal to Codspharium and Merismopedia ekganus. Susceptibility to thiobencarb the green algae. Strains of Pediastrum simpler were more sensitive than Pediastrum duplex and Chlorella saccharophlla was more sensitive than C. vu&Taris The high rate completely inhibited the growth of Stauia3bum paradawm and Chlamydomonus ova& The two rates of thiobencarb were lethal to all euglenophyceae. This study was examined the effect of thiobencarb herbicide on growth of phytoplankton community in rice field as important parameter in the evaluation of herbicide toxicity.