The author considers some methodological problems of interpretatio Graeca connected with the myth of Isis and Osiris in late antiquity. Taking as an example the plot of Isis’ «lost child», she demonstrates the peculiar way in which Egyptian mythological reality was reflected in the texts by Cassiodorus, Minutius Felix, Aelianus, Arnobius, Lactantius and in other sources. The interpretation of the «lost child» combines several lines of two mythological traditions: the slaughter of Zagreus, the murder of Osiris, Horus’ wound, as well as the search of the daughter (Persephone) or husband (Osiris). The «lost child» is a figure which developed as a result of combination of the Osiris myth with Greek myths of Demeter and Persephone and of Dionysus-Zagreus.