The scuticociliates Miamiensis avidus (syn. Philasterides dicentrarchi), Pseudocohnilembus persalinus, Pseudocohnilembus hargisi and Uronema marinum were cloned and identified using morphological characteristics and the small subunit ribosomal RNA gene (SSU rRNA). M. avidus strains YS1, WS1, YK1 and JJ3 from southern coastal areas and Jeju Island in Korea were pathogenic to olive flounder Paralichthys olivaceus (80 to 100% mortality in 8 to 10 g fish) when inoculated intraperitoneally (i.p.) with 1.0 to 1.4 x 10(6) ciliates fish(-1). Mortality was lower (10 to 45%) when the inoculum was 1.0 to 1.4 x 10(4) ciliates fish(-1) in the i.p.-injected group. The M. avidus strains of YS1, WS1, YK1 and JJ3 caused 60 to 100% mortality by immersion infection with 3.2 to 4.2 x 10(3) ml(-1) in 8 to 10 g fish and 3.0 to 4.0 x 10(3) ml(-1) in 30 to 40 g fish. M. avidus strain Mie0301 from the Mie prefecture in Japan caused 70% mortality by immersion infection with 4.4 x 10(3) ml(-1) in 30 to 40 g fish. The predominant sign was severe abdominal distension in i.p.-injected fish, and extensive ulcer lesions in the skeletal muscle in immersion-infected fish. Numerous ciliates were observed in the ascetic fluid, ulcers, haemorrhagic lesions, gills and brain of infected fish. However, P. persalinus (strain SCL-A), P. hargisi (strain SCL-B) and U. marinum (strain JK3) showed less than 30% mortality from both i.p. and immersion challenges, with no ciliate invasion in the skin, gills or brain. M. avidus-infected fish showed many ciliates in gills, fins, skin muscle, brain and intestine accompanied by necrosis and haemorrhages. However, no histological changes were observed in P. persalinus-, P. hargisi- or U. marinum-infected fish.