The electrochemical behavior of diquat at carbon paste electrode modified by fluoroapatite (FAP-CPE) was studied in 0.1 M K2SO4 using cyclic and square wave voltammetry. The diquat herbicide presented two reduction peaks, in -0.69 and -0.97 V, with profile of the totally reversible redox process. The influence of various experimental parameters on the FAP-CPE response was investigated (i.e. pH of electrolyte, carbon paste composition, accumulation time). The two reduction peaks showed that the peaks currents were found to be directly proportional to the herbicide concentration with detection limits (DL, 3 sigma) of 3.1 x 10(-8) and 7.6 x 10(-7) mol L-1 for peaks 1 and 2 respectively. In addition, the proposed methodology was applied in the river water samples, contaminated by diquat, without any pre-treatment step. Results indicated that the methodology could be employed to analyze diquat in such matrices. (c) 2008 Elsevier B.V. All rights reserved.