Thelohanellus kitauei was isolated from the koi Cyprinus carpio haematopterus, and the 18S rRNA gene of T. kitauei was amplified by optimised nested-PCR. The PCR product was sequenced and compared with other 18S rRNA genes of Thelohanellus species to investigate the relationships between their host specificities and infection sites. Based on the 18S rRNA sequences, T. kitauei is most closely related to T. hovorkai (which can infect the intestine). Phylogenetic analysis revealed that T. kitauei was clustered with other Thelohanellus spp. infecting Cyprininae. The present study suggests that the infection site and the host specificity (subfamily level) are reflected in the genetic relationships among Thelohanellus species.