Persistent toxic substances (PTSs) appear in most urbanized coastal areas of the world, accumulating in water, sediments, and biota, where they cannot be eliminated efficiently, and their derivatives can be highly toxic. Like many other countries, Turkey is also facing problems concerning pesticides and other PTS residues. BHC and alpha, beta, gamma and delta isomers of HCH were determined in 16 samples of surface and ground waters and mussels in the Middle Black Sea Region, and the concentrations of PCB and organochlorine (OC) pesticide residues were analysed in the eastern Aegean Sea water and fish samples. PCBs were not detected in Mullus barbatus living in the Aegean Sea, and its derivatives were detected in low levels. Thirteen OC pesticides were determined in water, sediment, fish, and water birds in the Goksu Delta. Thirteen OC pesticides were analysed in the Sariyar Dam Lake, in Sakarya basin, five lakes in Central Anatolia, and the Meric, Delta in water sediment and fish samples. Some of these pesticides were found in more or less all of the samples. Micro- organisms in soil and some saprophyte fungi detoxify chlorinated hydrocarbons by dechlorinating them. The toxicity of degraded metabolites of these insecticides, for which the Cl amounts have been reduced, decreased consequently, and toxic effects also reduced in mice when Cl was removed. Mixed cultures of micro-organisms isolated from agricultural soils were used to degrade endosulphan, carbaryl, and malathion. This result indicates that these chemicals break down in soil, resulting in a significant decrease in its toxic effects. Toxic substances cause acute, sub-chronic, and chronic effects in the environment on biota. The hermit ibises, an endemic migratory bird species, no longer visit the Birecik district. In Turkey, the reason for this is also thought to be the usage of organochlorine pesticide sprays in agricultural areas of the district. In recent years, studies have been conducted on the activation of PTSs and Cyt P450 mono-oxygenase enzymes in indicator fish species. One of these studies has been conducted on the bottom-feeding fish that live in Izmir Bay. The outcomes showed a significant decrease in the EROD activity in fish ( mullet and the benthic fish common sole are used as biomonitors for PAHs and PCB-type pollutants) that have been harvested from six different stations along the Bay from the city borders towards the inner parts of the Aegean Sea. The effects of OCPs have been determined by other studies in Sariyar Dam Lake and Meric, Delta. An increase in the EROD activity has been determined in fish living in these environments.