Abstract. – Norway lobsters (Nephrops norvegicus) with epizoans were held in tanks with seawater and 14C-labelled microalgae (Rhodomonas baltica) for 24 h. The isotopic activity of two epizoans on the mouth parts of the host, the bryozoan Triticella flava and the cycliophoran Symbion pandora, were compared by liquid scintillation counting. There was no isotopic activity recovered in Symbion pandora but Triticella flava actively ingested the labelled algae and accumulated isotopic activity over time. The cycliophoran feeding activity was assessed further by direct observation of detached feeding stages in a recirculation system. Feeding stages did not respond when algae were offered, but immediately began feeding when diluted hemolymph or homogenized mussel was offered. The results explain why cycliophorans only occur on the mouthparts of the host and support the notion that cycliophorans are obligatory commensals that solely depend on the food provided by their host. Cycliophorans start feeding when the host begins to feed and high concentrations of food particles become available. It is likely that cycliophoran feeding individuals have developed sensory mechanisms to synchronize their food supply with the feeding activity of the host but the precise mechanism is still unknown. In contrast T. flava is a facultative commensal that can sustain on plankton independent of host feeding activities. In agreement with this T. flava often occurs on other parts of the host than the mouth parts