Abstract: Water pollution with chlorinated pesticides is one of the most serious environemntal problems due to their highpersistence as a result of the slow biodegradation. Residue levels of organochlorine compounds including (P, P' -DDT, y-HCH and DDE) and cyclodiene components (aldrin, endrin) in the water and sediments of Lake Mariut, Alexandria (brackish water) were analyzed and determined by capillary gas chromatography. Bacterial isolates collected from sediments of such lake were identified and investigated for their ability to biodegrade the selected pesticides. Water and sediment samples were collected from six different sites in the main basin of Lake Maruit and also through three successive seasons, summer, autumn and winter 1996-97. Bacterial isolates were identified and subjected to two concentrations: 0.05 and 50 ppm of the investigated pesticides to study the interaction between pesticides and bacteria. Results showed that lindane, aldrin. P,P' - DDT and endrine were present in the water and sediments of Lake Mariut at very high levels with residue levels significnatiy higehr in sediments compared to water samples. Seasonal and spatial variation of their distribution in the water and sediments were observed. Biodegradation results showed superior ability of the isolated bacteria to decompose the investigated pesticides with very high efficiency reaching 100% for most of them. Results also revealed selective ability among the tested bacteria for biodegradtion of different pesticides especially at the lowet concentrations.