Hemorrhagic thelohanellosis in two varieties of carp (Cyprinus carpio) caused byThelohanellus hovorkai was investigated by experimental infection. One-year old coloured carp (koi-carp) and common carp were kept for five weeks in tanks with oligochaetes (Branchiura sowerbyi) collected from a pond where hemorrhagic thelohanellosis was enzootic. Another group of coloured carp was exposed only to the effluent from a tank containingB.sowerbyi. Four weeks post-exposure, both coloured carp and common carp maintained withB. sowerbyishowed clinical signs of infection and chronic mortalities. On the other hand, neither mortalities nor clinical signs were observed in the effluent group, though light infections withT. hovorkaiwere detected. Control groups of coloured carp and common carp maintained in clean tanks showed no signs of infection at all. Through a standardized trypsin digestion method, the intensity of infection withT. hovorkaiin carp maintained withB. sowerbyiwas determined to be more than 50, 000 spores/g-tissue. Histological observations revealed thatT. hovorkaiplasmodia were most frequently detected in connective tissues of the intestine and the ventral skin in the cohabitation group, whereas they were found most frequently in the gills of the fish of the effluent group. In conclusion, hemorrhagic thelohanellosis was experimentally induced in carp by maintaining hosts withB. sowerbyiregardless of host's varietal difference, and the ingestion ofB. sowerbyiinfected with the actinosporean may be an important factor inducing the disease conditions.