Abstract: The effect of parathion and its degradation products on photosynthesis of Scenedesmus obliquus was investigated. The toxicity of p-nitrophenol is much stronger than the sodium salts of nitrophenol, diethylthiophosphate and parathion. Studies of simulated algae-bacteria system of Pseudomonas sp. CTP-02 and Scenedesmus obliquus, using p-nitrophenol as substrate, indicate that oxygen needed for aerobic bacteria can be provided by photosynthesis of algae.