Abstract: Pollution of the aquatic environment by petroleum and its products is common the world over. This study is aimed at examining sublethal effects of petrol and diesel on enzymes in Tympanotonus fuscatus namely: Aspartate Transaminase (AST) (E.C. 2.6.1.1), Alanine Transaminase (ALT) (E.C. 2.6.2.2) and Alkaline Phosphatase (ALP) (E.C. 3.1.3.1) activity after exposure. The periwinkles were exposed to 10.40, 15.60, 21.00, 26.00 ml L-1 and a control. The organs were removed on the sixth day and were prepared for enzymatic analysis. Enzyme activities were compared to the control value and between the toxicants. The effects of the toxicants on AST activity in the muscle and viscera were significantly different (p>0.05) from the control value (137.50±15.10 IU L-1). AST activity were raised more in petrol concentrations than the diesel concentrations in the muscle. The reverse was the case in the viscera at 15.60 ml L-1 (227.50±24.75 IU L-1). ALT activity in the muscle were not significant (p>0.05) between the toxicant media. In the viscera, significant differences (p>0.05) were observed in some of the concentrations with petrol showing higher activity. ALP activity in the muscle were not significant (p>0.05) in both media, but were more elicited in the diesel concentrations. In the viscera, the activities of ALP were more pronounced in the petrol concentrations and were significant (p>0.05) at the higher diesel concentrations. The exposure of Tympanotonus fuscatus to petrol and diesel concentrations caused changes in the enzymatic activities in the organism with those of petrol more pronounced than those of the diesel.