Abstract High-precision oxygen isotope analyses were carried out on dissolved phosphate extracted from discharge waters from three wastewater treatment plants (WTP) located in western France, as well as on the different phosphate-based fertilizers applied by farmers in the same region. Measured δ 18O values of phosphate from chemical fertilizers range from 19.6 to 23.1‰, while those of phosphate from WTP discharge waters are more tightly grouped between 17.7 and 18.1‰. The variablility in δ 18O values of phosphate fertilizers is attributed to oxygen isotope variations of the phosphorite deposits from which France's fertilizers are manufactured. The significance of the δ 18O values of phosphate from WTP discharge waters is less straightforward. At present, it is not clear whether these values are primary isotopic compositions corresponding, e.g., to the oxygen isotope composition of phosphate builders included in detergents ( δ 18O P=17.9‰), or represent secondary values reflecting biological recycling of the phosphate in equilibrium with ambient WTP water The restricted difference in isotopic composition obtained between phosphate from fertilizers and phosphate from WTP discharge waters (<2‰), as well as the fairly large internal isotopic variability observed in both end-members (⩾1.5‰), cast doubt about the possibility that the oxygen isotope composition could serve as a tracer for the source of anthropogenic phosphates in waters.