Juveniles of Polyphemus pediculus (Linnaeus, 1761) emerge into the aquatic environment from resting eggs at an earlier stage of development, compared to those that hatch from subitaneous eggs. They become sexually mature after five moults, while individuals originating from subitaneous eggs become mature after four moults. The numbers of moults under both circumstances are strictly stable. Morphometric characteristics, body length, and the ratio of the entire body and its parts change with every moult, but the intensity of these changes is different in individuals originating from different types of eggs. Newborns originating from resting eggs, are larger, but have less setae on antennae I-II and on the third endopodal segments of thoracic limbs I-III. Morphometrically they are closer to sexually mature individuals of Polyphemus exiguus Sars, 1897, than to immature individuals of P. pediculus originating from subitaneous eggs. Their morphometric correspondence to mature individuals of the freshwater form of their own species, P. pediculus is even less. As a consequence, the numbers of setae on the first segments of the endopodites of the thoracic limbs can not be used as a systematic character for the species of the genus Polyphemus.