The Polychaete Neridid, Hediste (Nereis) diversicolor, isone of the soft bottom worms that were widely used as a biomarker of pollution in the coastal habitat. The present study was designed to assess the effect of the widely used pesticide, Malathion, on the protein profile of H. diversicolor, existing in an Egyptian polluted lake (Lake Burullus) connected to the Mediterranean coast by the Burullus outlet. SDS-PAGE technique was applied to detect polymorphic protein bands under different concentrations of Malathion in both worms and sediments. The results indicated a positive correlation between increasing temperature and polymorphism percentage of worm's protein bands and the Malathion content. The highest protein polymorphism (42%) was detected at high temperatures (30^°C), whereas the lowest polymorphism (7%) was detected at low temperatures (17^°C). Similarly, the highest concentrations of Malathion in the worm and sediment were recorded at 30^°C and the lowest concentrations at the lowest temperature. The present study revealed also that Malathion level in the worms (4.17 - 4.4 ppm) was higher than in the sediment (3.14 - 3.87 ppm).