A very sensitive flow injection method with spectrophotometric detection has been developed for the on-line determination of copper in natural waters. The method exhibits a limit of detection three times lower than the most sensitive direct spectrophotometric method previously described and then allows the direct and simple in situ determination of copper in most natural waters. The method was based on the measurement of the absorbance of the coloured complex formed by copper with the chromogenic reagent di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an alkaline medium. This complex presents stoichiometry 1:2 (Cu:dPKBH), and exhibits maximum absorbance at 370nm. The manifold used was very simple, and consisted of two channels. The first one contained the sample while the second one contained the colorimetric reagent (3.3x10(-4)M dPKBH in 10% ethanol), in a 1.6x10(-2)M phosphate buffer solution at pH 8. The performance of the system was optimised by using both univariate and modified simplex methodologies. When modified simplex was used, the best signal was obtained for a sample injection volume of 529mul, a reaction coil length of 1.29m, and a reagent flow rate of 4.8mlmin(-1). Under optimum conditions, the response was linear up to 3mgl(-1) copper, the equation of the straight line being y=0.314x+5.2x10(-4) (r(2)=0.998). The method allowed a sampling frequency of 40 samples per hour and exhibited a precision of 2.11% (as R.S.D., n=11). The limit of detection was 4.6mugl(-1) (calculated as 3s(b)/m, where s(b) is the standard deviation of the y-intercept and m represents the slope of the straight line), and was therefore more sensitive than all the direct continuous methods reported previously. The method was successfully applied to the analysis of real water samples, with an average relative error of 5.32%.