We used nucleotide sequence data from the ribosomal RNA (rRNA) gene cluster to determine the affinities of rhizocephalans reported to be Sacculina carcini which parasitize different portunid hosts: the green crab Carcinus maenas from Sweden; C. maenas from England; Liocarcinus marmoreus from Ireland; and Liocarcinus holsatus from Wales. There were no differences in nucleotide sequence data from the 3′ region of the small subunit (SSU) (160 bp) and internal transcribed spacer 1 (ITS1) (274 bp) rRNA of these parasites. The same region was sequenced from Sacculina granifera from Queensland, Australia, parasitic on Portunus pelagicus; Sacculina oblonga from Kyushu, Japan, parasitic on Cyclograpsus intermedius; and an undescribed rhizocephalan from Victoria, Australia, parasitic on Nectocarcinus integrifrons. The consensus nucleotide sequence of S. carcini from the ITS1 rRNA differed from S. granifera by 52% (146/282 sites), from S. oblonga by 56% (162/289 sites) and from the rhizocephalan from N. integrifrons by 33% (94/284 sites). The consensus nucleotide sequence of the 3′ region SSU rRNA of S. carcini and the rhizocephalan from N. integrifrons, which included the V9 domain, differed from S. granifera and from S. oblonga by 6% (9/160 bp). The high divergence rate of ITS1 rRNA sequence between species of Sacculina makes it a valuable diagnostic tool for parasites in this genus. On that basis, the nucleotide sequence data suggest that S. carcini infests at least two genera of crabs from a broad geographic distribution. This may limit the use of this parasite for biological control of introduced populations of green crabs.