- Summary Tetrahymena is a ciliated protozoan that can infect a wide range of fish species, although it is most commonly reported in guppies ( Poecilia reticulata ). The aim of this study was to compare the susceptibility to infection with Tetrahymena of five different ornamental fish species from two different super orders. The species examined were platy ( Xiphophorus ), molly ( Poecilia sphenops ) and angelfish ( Pterophyllum scalare ) of the Acanthopterygii super order (which also includes guppies) and goldfish ( Carassius auratus auratus ) and koi carp ( Cyprinus carpio ) of the Ostariophysi super order. These two super orders are phylogenetically distant from each other. Infection with Tetrahymena resulted in parasite invasion of internal organs, skin and muscle in all fish species. A relatively strong inflammatory response was observed in infected goldfish and koi, with negligible response in fish species of the Acanthopterygii super order. Guppies were the most susceptible to Tetrahymena infection, exhibiting a mortality rate of 87% and 100% in two separate experiments. A high mortality rate was also observed in platy (77%), while that of molly and angelfish was significantly lower (23% and 33%, respectively). Goldfish and koi carp were less susceptible to infection compared with guppies (24% and 59% mortality, respectively). Immunization studies revealed that the Tetrahymena are immunogenic, since infection of koi carp increased their Tetrahymena immobilization response by approximately three-fold at 3 weeks post infection, while immunization with Tetrahymena plus adjuvant increased their immobilization response by approximately 30-fold.