Summary.Combinations of copper sulphate pentahydrate (CSP) at 1 0 ppmw of copper with diquat, paraquat, ametryne, atrazine, terbutryne or 2,4‐D at certain concentrations and contact times increased the copper content in hydrilla (Hydrilla verticillataGasp.) in comparison with CSP alone. Combinations of CSP with fenac, dichlobenil, diuron, an amine salt of endothal or dinitrophenol had no effect on copper uptake. CSP plus 1·0 ppmw of ametryne exhibited a synergistic response on hydrilla as determined by dry weight, and was associated with a high concentration of copper. Phosphorus levels in hydrilla were reduced by diquat and paraquat. Hydrilla treated with CSP plus 0·1 ppmw of paraquat contained 32·4% less phosphorus than plants treated with paraquat alone. The phytotoxic effect of certain of these combinations on hydrilla may be due principally to an increased accumulation of copper and a reduction in phosphorus con