The aim of this study is to examine the activity of several biomarkers in carp, Cyprinus carpi L., to determine their response to xenobiotics, such as organochlorine pesticides (OCPs), in the Meric Delta. Fish were collected from contaminated sites and from areas regarded as relatively less contaminated in four sampling periods. Hepatic glutathione S-transferase (GST), carboxylesterase (CaE), lactate dehydrogenase, aspartate aminotransferase, and acid phosphatase activities were measured as biomarkers in the fish. For all fish, the condition factor and hepatosomatic index, were calculated to determine the condition of the fish. The results of this study indicated that the mean GST activity showed an increase in fish from the Meric-Ergene junction site and a decrease in Enez site with respect to fish from Meric site. Furthermore, the study shows that spatial and temporal changes of biomarkers such as GST and CaE might be useful for the assessment of environmental contamination in the Meric Delta. (C) 2012 Elsevier B.V. All rights reserved.