The biokinetics of Cu in Daphnia magna, including dissolved uptake, assimilation and efflux, has been determined using a gamma 67Cu radiotracer methodology. However, this gamma emitting radioisotope is not readily available due to its very short half-life. In thepresent study, we employed a stable isotope tracer (65Cu) to determine the Cu biokinetics and compared our results to those determined increasing food concentrations from 2xl04 to l×l05 cells/mL, the Cu assimilation efficiency (AE) decreased from 46% to 11%,compared to a decrease from 27% to 16% when determined using 67Cu. The efflux of Cu from Daphnia magna was quantified following both dissolved and dietary uptake. The efflux of waterborne Cu was comparable to that of dietborne Cu and the efflux rate constant (0.32-0.52 day-1) was higher than that determined by 67Cu (0.19-0.20 day-1). By considering different water properties and handling procedure between the two experiments, we believe that these differences are reasonable. Overall, this study demonstrated that the enriched stable isotope tracer technique is a powerful tool to investigate metal bioavailability and maybe a good alternative to radioactive measurements.