A 48 h LC50 values of (2,4-dichlorophenoxy)acetic acid (2,4-D), a widely used agricultural herbicide, were determined on the larvae and adult Nile tilapia (Oreochromis niloticus L.). Each test was repeated three times. The data obtained were statistically evaluated by the use of the E.P.A computer program based on Finney's probit analysis method and a 48 h LC50 value for Nile tilapia (O. niloticus L.) larvae and adults were found to be 28.23 mg/L and 86.90 mg/L, respectively in a static bioassay test system. 95% lower and upper confidence limits for the LC50 were 22.55-32.98 and 80.67-92.80 mg/L, respectively. Water temperature was 24 ± 1°C. Behavioral changes of both tilapia life forms were examined for various herbicide concentrations. © 2005 Elsevier B.V. All rights reserved.