Tests were performed to determine the acute toxicity of simazine, a herbicide commonly used in pond aquaculture, to the cIadoceran Daphnia pulex and to 3- and 7-day old larval striped bass (Morone saxatilis). The 48-hour LC50 for 3-day-old striped bass was r8 mglIiter in hard water (220 mg/liter as CaC03 ) and r6 mglIiter in soft water (120 mglIiter as CaC03 ); corresponding estimates for 7-day-old larvae exceeded roo mg/liter. The estimated 48-hour LC50 for D. pulex exceeded 50 mg/liter in both hard and soft water. These estimates were considerably higher than recommended pond application rates of simazine, but abnormal behavioral responses of both test organisms to sublethal concentrations suggested that long-term exposure could be harmful. Inasmuch as 7-day-old larvae were far less sensitive than 3-day-old larvae, the timing of simazine applications in relation to age of fish could have pronounced effects on survival of striped bass in ponds.