1.The present contribution deals with the biology and control of a Bothriocephalid tapeworm which is infectious to the young grass carp(Ctenopharyngodon idellus C. V.)and consequently causes high mortality among the hosts. 2.The parasitism is,hitherto,found as a localized disease,serious at places along the Pearl River,especially in the lower basin,where the young fish nursery industry is highly developed and concentrated.None has been recorded in fish nursing areas along the Yangtze River. 3.The incidence of infection in early nursing and wintering periods was 57.6 and 65.3% respectively.Results of examination of 405 young Ctenopharyngodon taken from 7 ponds from Chiuchiang Fish Nursery Station in 1953 show an incidence of infection of 57.6%(1.6—100%).The number of plerocercoid forms predominate over the adult.The winter census in 1954 covers the examination of 3037 young Ctenopharyngodon taken from 30 out of the 1060 nursing ponds in three districts in Chiuchiang,Kwangtung. Only one pond proved free from infection;the rate of infection in the other 29 ponds averaged 65.37%(20—100%).The highest record of cestode-burden in a single fish registers 467 tapeworms in the fore-intestine.The mortality rate in winter may reach as high as 90%.All materials examined were taken by random sampling. 4.Eggs when laid are partly embryonated and partly segmented.The proportion varies with seasons:fully embryonated eggs may reach 89% from April to October(24—29℃)but only comprise 2% from November till February(14.8—21.3℃). 5.Temperature shows a marked influence upon the incubation period. The rate of hatching at 28—30℃ is highest in the first day,averaging 76.9%, and the incubation period extends to 5 days.Hatching becomes less regular below 25℃.Incubation period ranges 10—28 days at 14—15℃.There is little hatching below 12℃ and none above 37℃. 6.Eggs exposed to a relative humidity of 95% prove non-viable within less than one hour.Eggs remain viable in substrate moisture of 12.3% for 3 days and of 17.2% for 5—7 days.The minimum amount of substrate moisture necessary for development of eggs is found to be above 10%. 7.The distribution of coracidium in water shows highest density within a range of 0.2 cm.along the substratum. 8.Fresh-water cyclops,Mesocydops leuckarti Claus,Thermocyclops taihokuensis(Harada)and Ectocyclops phaleratus medius(Kiefer)are readily infectible with procercoid,but not Calanoids and Cladocera.Rate of natural infection may reach 7%. 9.Procercoid becomes mature in 4 days at 25—33℃,in 5 days at 20.3℃. Linear growth at 13.8℃,as illustrated by size-group frequencies in consecutive days,shows bimodal distribution,in which 48.9% of procercoid reached maturity on the 21st day,whereas others remained small in size and never came to maturity.Density of infection in cyclops is not an important influence on the growth and maturity of the procercoid. 10.The longevity of infected cyclops shows a maximum of 49 days in 16.2℃(averaged water temperature in December-January);35 days in 20.5℃(March-April)and 11—18 clays in 29—31℃(July-November). 11.Plerocercoid stage in the fish fore-gut lasts 9 days when proglottisation begins.Plerocercoids show very slow linear growth during the first 9 days in the host,as compared with the rapid growth during the 10 to 21 day period.The first discharge of eggs occurs between the 21st and the 23rd day in 28—29℃. 12.The growth form of population of the tapeworm within the host appears in three stages:(1)the establishment of population;(2)the period of rapid growth and(3)the period of negative growth.O-group Ctenopharyngodon never acquire immunity and infected hosts are subjected to reinfection. Large population densities of tapeworms within the host inhibit the maturity and growth of the cestode itself and also the growth of the host.On the other hand,high proporion of adults occur in small population densities and some of the adult reach their maximum growth.Mortality of the host is highest when its body length is less than 80 mm.Population of the tapeworm begins to decline when the body length of the young fish exceeds 100 mm.Fishes beyond 1-year group are rarely infected with this tapeworm,and none of the worm is found to possess reproductive potential.The stage between establish- ment of population and declination covers about one year.Change of the vegetative food for the host shows no marked influence on the decline of the worm population. 13.Other Cyprinids readily infected with this bothriocephalid tapeworm are:Mylopharyngodon aethiops(Basileusky),Hypophthalmichthys molitrix(C. V.Cirrhina chinensis Gunther,Luciobrama macrocephalus(Lacépède), and Elopichthys bambusa(Richardson).None of the species is found to be so seriously infected as those of the young Ctenopharyngodon. 14.The effect of parasitism on the host is shown in the following diagnosis:(1)Loss of body weight.The correlation between the body length and the body weight of the fish is expressed in the following allometric equations: Infected case:log Y= .6434+2.4457 log X Superinfected case:log Y= .7095+2.2532 log X Uninfected case:log Y= .3240+2.9870 log X;(2)Pernicious anemia.Red cell count dropped from 8.5×400000 to 4.8×400000;(3)Chronic inflammation of the intestine.Presence of large mononuclear phagocytes and large number of fibroblasts in the thickened Tunica propria and also the infiltration of lymphocytes and large mononuclear phagocytes between circular and longitudinal muscles of Tunica muscularis. 15.Treatment with areca nut in the laboratory decreases the rate of infection from 4.8—45%. 16 Method of control:either 1)by killing the egg of the tapeworm and cyclops in the nursery pond after the annual clearance of young fishes in Spring with 1/2000 concentration of lime or 50 p.p.m.bleaching powder; or 2)simply by keeping the pond vacant for 50 days after the annual clearance. The length of the interval is based upon the incubation period of the egg and the longevity of the infected cyclops under known conditions.Both methods prove effective in large-scale field appliance.