Abstract <p>Rare earth elements, including lanthanum, are actively involved in various industries and enter the environment. The aim of this study is to compare the bioaccumulation of La(III), Cu(II) and their combination by adult and juvenile <i>Daphnia magna</i> Straus in chronic tests. Daphnia were grown for 40–50 days in drinking water and in water supplemented with La(III), Cu(II) and their combinations (equimolar concentrations of 0.00008, 0.0016 and 0.0008 mmol/L). Juveniles were collected daily, and adults were collected at the end of the experiment. The content of elements in the biomass was determined by the atomic emission method with inductively coupled plasma (ICP). <i>D. magna</i> actively extracted La from clean water: The BAF for adult individuals was 5 times higher for La than for Cu, and 8 times higher for juvenile individuals. In polluted waters, a “jump” in the bioaccumulation of metals at a level of 0.008 mmol/L was shown. For example, the La content in the presence of Cu in adult <i>D. magna</i> exceeded 200 μg/g. A feature of juvenile individuals was the rapid accumulation of elements: in 24 h they accumulated La(III) and Cu(II) at the same level as adult individuals in 40–50 days. Thus, new patterns of La and Cu extraction from water by hydrobionts are shown, which are especially necessary for the development of scientifically based standards for La content in water.</p>