There are numerous open questions concerning the life cycle of horsehair worms (Nematomorpha), especially about the parasitic phase. Complete development takes place only in a few host species. These hosts, almost exclusively arthropods, may be infected directly or indirectly through a paratenic host which carries larvae and cysts of hairworms and was infected in an aquatic habitat. Even though different aquatic animals have been cited as paratenic hosts, little is known about the viability of the larvae or the cysts inside them. On the basis of experimental infections we demonstrate that the larvae entering a paratenic host (Physella cubensis) do not encyst immediately and that larvae and cysts can infect a second paratenic host (Jenynsia multidentata).