Two types of infectious agent within rootlet cells of the parasitic barnacle, Sacculina carcini have been recognized by transmission electron microscopy. The rootlets were dissected from the common shore crab, Carcinus maenas, collected from two locales—Plymouth and Pwllheli. Yeast cells were identified within cells of S. carcini rootlets from crabs collected at both locations and an iridovirus was also found, but only in rootlets from Plymouth crabs. These infectious agents were never found co-occurring in the rootlets from Plymouth crabs. Both agents, when present in rootlets, were also present in the respective host crab tissues. It is therefore concluded that S. carcini rootlets are susceptible to invasion from natural infectious agents of the host crab.