A 96-h LC50 values of 2,4-D [(2,4-dichlorophenoxy)acetic acid], a common contaminating agricultural herbicide, were determined on the adult common carp (Cyprinus carpio L., 1758; Pisces, Cyprinidae). The study was conducted in two stages using 130 carp. The data obtained were statistically evaluated by the use of the EPA computer program based on Finney's Probit Analysis Method and a 96-h LC50 value for C. carpio L., 1758 was found to be 63.24 mg/l in a static bioassay test system. 95% lower and upper confidence limits for the LC50 were 55.03 and 71.92 mg/l, respectively. Water temperature was 17 ± 1 °C. Behavioral changes of the above mentioned species were examined for various herbicide concentrations. © 2003 Elsevier Science Ltd. All rights reserved.