Leopard coral grouper, Plectropomus leopardus, is a highly valued fish and the research on its seedling production has been conducted at Yaeyama Laboratory, Seikai National Fisheries Research Institute (SNFRI), Okinawa Prefecture, Japan since the 1980s. A sporadic infection with the capsalid monogenean Neobenedenia girellae was reported from broodstock before (Ogawa et al., 1995a), but it was not a serious obstacle to breeding activities. However, since 2016, after the initiation of a larger scale seedling production and following year around grow-out rearing, mortalities in juvenile groupers have often been associated with heavy infections of capsalid monogeneans on the fins and skin. Although freshwater bathing, a common deworming practice against capsalid monogeneans (Ogawa and Shirakashi, 2017), was effective for dislodging the flukes from fish, frequent deworming treatment requires much labor. Moreover, P. leopardus is vulnerable to low salinity, and the freshwater bathing was sometimes harmful to infected groupers (Mizuochi, unpublished observation).We started studies to establish safe and effective control measures against the parasite infestation. During our investigations, it was revealed that this parasitic disease was caused by a capsalid different from N. girellae. In the first step of our study, we attempted to identify and describe the monogenean. Materials and Methods Sample collection, preparation and morphological analysesLive capsalid monogeneans (n = 19) were collected under a stereomicroscope from the fins and skin of leopard coral grouper Plectropomus leopardus reared at SNFRI, Okinawa Prefecture, Japan. Additionally, two specimens of apparently the same species of capsalid dislodged by a freshwater bath were collected from wild P. leopardus caught near Ishigaki Island.Specimens were flattened between a glass slide and coverslip, fixed in AFA (mixture of 70% ethanol: 20

The leopard coralgrouper, Plectropomus leopardus is a promising culture target and its culture research has been conducted since 1985 at Yaeyama field station