Nauplii of the rhizocephalan families Peltogastridae and Lernaeodiscidae carry a torus-shaped collar around the body. It consists of an exceedingly thin cuticle connected to the general body cuticle along a continuous narrow ridge. In nauplii of some species, the collar is very large and its surface ornamented by a very conspicuous reticulated pattern of ridges. In other species the collar is smaller and with a smooth surface that impedes its detection when using a light microscope. The collar is absent from nauplii of all investigated species of the Sacculinidae. Transmission electron microscopy shows that the collar of the succeeding nauplius instar is formed in an unexpanded state beneath the old cuticle and it must therefore be inflated at or immediately after ecdysis. At ecdysis the collar of the old instar breaks along the attachment ridge, leaving the empty collar and the exuvium of the general body as separate objects. The collar must have a profound influence on the hydrodynamic properties of the nauplius, both when swimming and passively sinking. We therefore consider it as a floatation device, a view supported by the absence of the collar in the rapidly swimming cypris larvae. There is no obvious homology to the collar outside the Rhizocephala, and it therefore appears to have evolved only once.