Glucose amendment (0.5%) to the culture medium of Chlorella vulgaris reversed the algal toxicity caused by p-nitrophenol (PNP) and m-nitrophenol (MNP), only at their algistatic levels. Algistatic and algicidal concentrations of PNP and MNP, in the absence of glucose, led to the liberation of more nitrite in the culture medium. However, addition of glucose inhibited nitrite formation.