Toxic effects of water soluble fractions (WSF) of four different fuel oils on a microalga. Tetraselmis gracilis, were examined and compared. On applying different concentrations of WSF, a decrease in cell population was observed. Depending on different physico-chemical properties, the hydrocarbons showed different inhibitory effects on the growth of Tetraselmis gracilis. Petrol was the most toxic and produced inhibitory effects even at low concentrations while HDD and crude oil inhibited growth at higher concentrations. Kerosene did not appreciably alter the growth either at lower or higher concentrations. The increase in toxicity was attributed to the presence of aromatic hydrocarbons of medium to higher molecular weight and their chemical modifications. The experimental data has shown that different oils have different potential for environmental damage, depending on the types and concentration of soluble and dispersed hydrocarbon present in the WSF