Two-way ANOVA proved that different concentrations of water extract of diesel and gasoline fuels significantly (p ≤ 0.01) affect the growth and biochemical activities of Dunaliella salina. The effects were found to depend mainly on type, concentration of the extract and period of culturing. Low concentration of both extracts (25%) ameliorated the growth and most of the measured biochemical analyses (pigment fractions, carbohydrates and protein). On the contrary, concentrations higher than 25% decreased these measured parameters. The toxic effect of these extracts was increased by increasing their concentration. The results proved also that gasoline is more toxic to D. salina than diesel fuel oil.