The water of Nif Brook (Izmir, Turkey) is polluted by industrial, domestic, and agricultural sources. As the water of the brook is used for domestic and industrial water supply as well as for irrigation, it is of great importance to know the toxicity potential for the natural populations. For this purpose the standard test. protocol for the short term phytotoxicity test method OECD 201 Algal Growth Inhibition has been evaluated with cultures of green algae Desmodesmus (=Scenedesmus) subspicatus as the representative of the first trophic level. The test has been used to assess the toxicity of both water and sediment samples from the brook. Five sampling sites from the brook were selected considering the type of pollution sources. The water samples were tested with 10, 20, 40, 60, 80 and 100 % dilutions in a test medium. Extracts of sediment samples were assayed in three different concentrations (20, 40 and 100 mg/mL). The effects on the growth of D. subspicatus were evaluated by scoring cell numbers under the light microscope with a Neubauer haemocytometer counting chamber. According to the results, the water samples stimulated the algal growth (except station 1); although all sediment samples inhibited the growth of populations, in several grades.