Ichthyophthirius multifiliis(Ich)is one of the most pathogenic ciliated protozoan parasitizing the gills and skin of fishes,and can cause mortality of a large number of cultured fishes,subsequently resulting in a serious economic loss in aquaculture.So it is necessary to control the parasitic ciliate in aquaculture.In the past several decades,malachite green was frequently used to treat ichthyophthiriasis in fish farms.However,malachite green has been banned nowadays for use in food fish due to its carcinogenic and genotoxic effects on humans.So far,there is none of both effective and safe therapeutants to treat Ich,therefore,there is an urgent need to find both efficacious and safe parasiticides to control Ich.Previous studies showed some Chinese medicinal plants contained anti-Ich compounds,which were easily degradable,and safe for human and water environment,and could further be developed into promising therapeutants against Ich,but the anti-Ich activity of Cinnamonum cassia Presl,a Chinese medicinal plant,was unknown.The present study was designed to investigate the anti-Ich activity and compound of C.cassia Presl with grass carps under in vitro conditions.The powdered C.cassia were extracted primarily with 95% ethanol.Then the crude extract was sequentially extracted with petroleum ether,ethyl acetate and methanol,respectively.The anti-Ich efficacy of petroleum ether fraction was the best among the 3 extracts.Thereby,bioactivity-guided fractionation and isolation of the compound responsible for anti-Ich activity were carried out with the petroleum ether extract,and one purified compound with anti-Ich activity was obtained,and subsequently identified as cinnamaldehyde by means of MS,¹H NMR,¹³C NMR spectroscopic analyses.The anti-Ich compound,cinnamaldehyde,could cause 100% mortality of trophonts at 50 mg/L,and theronts at 8 mg/L for 4 h exposure.The 4-h EC₅₀ values of cinnamaldehyde against trophonts and theronts were 13.9 mg/L and 1.8 mg/L,respectively.The compound also stopped reproduction of all encysted tomonts at 50 mg/L.These data confirmed that cinnamaldehyde is effective against I.multifiliis,and promising for development of a new,safe and efficacious antiparasitic agent.