Metallothioneins (MTs) are small cysteine-rich proteins that play important roles in metal homeostasis and protection against heavy metals; therefore, they were used as biomarkers to evaluate metals pollution in the aquatic ecosystem. The present work aims to evaluate the tissue immunohistochemical expression of MT in Biomphalaria alexandrina snails and Oreochromis niloticus fish collected from Lake Manzala in the area of Port-Said, Dakahlia and Damietta governorates in response to the recorded levels of Cd, Cu, and Pb in their aquatic habitats. The levels of the heavy metals; Cd, Cu &Pb in water samples from Port Said were significantly exceeded those of Dakahlia (p<0.01), also, a similar pattern was recorded for Cu levels between Damietta and Dakahlia (p<0.01). Accordingly, the tissue expression of MT was significantly increased in snails and fish specimens from Port Said and Damietta compared to Dakahlia governorate specimens (p<0.01 and p<0.05 respectively). In general; MT immunopositivity in snails regarding the number of MT-positive cases and the number of positive cells, was observed as digestive gland> prostate tubules> ovotestis>  head-foot area and in fish, it was as follows: liver & spleen> gills > ovary. Conclusion: There is a positive correlation between heavy metals levels and MT expression in snails and fish collected from Port Said, Damietta and Dakahlia, El-Manzala Lake (p<0.001& r=0.636, p< 0.001& r=0.375 and p< 0.05& r=0.296, respectively), hence MT could be considered as a good potential biomarker for toxicological studies and for assessment of heavy metal stress in aquatic environment.