Fenoxaprop-p-ethyl, a herbicide that is commonly used in Cukurova region, was studied for possible adverse impacts on the Seyhan dam plankton and water quality variables in laboratory microcosms for 40 days. Water containing natural plankton, and sediment were collected from the Seyhan dam. The herbicide was added to six microcosms in 0.1, 1, and 10 mg L-1 concentrations. Two microcosms were used as reference groups. The results indicate that pennat diatom, Cladocera, and Copepoda at 10 mg L-1 contaminations and centric diatom and Chlorophyta at 1 mg L-1 contamination were affected. This herbicide did not alter water quality.