The adverse effects of the rice herbicide thiobencarb (Saturn) on rice field chlorophyll "a" concentrations, =plankton communities and water quality were studied throughout the period investigation. Treatments simulated direct spraying of rice field by this herbicide was followed. Two rates of 1 and 1.5 kg faddan'^l of thiobencarb were tested, representing approximate high and low level. Water quality measurements were done prior to herbicide application (0 time) and then after 1, 2, 3, 4 days from application. The obtained results indicated that Thiobencarb was reduced greatly oxygen production in water. These stressful oxygen levels, however, occurred only with the simulated direct spraying by this herbicide in rice field. Also, stimulation of chlorophyll ^-a^- was recorded. Herbicide concentration froued influenced in this respect. In addition, significantly higher rotifers and copepods counts were observed after 4 days from herbicide treatment.