Abstract: This paper discusses the relation between the constitution of different chemicals and their cer- caricidal activity.The tested chemicals are chlorinated phenols,benzoic esters,substituted benzoic esters,halogenated acetic esters,thiocyanated acetic esters,2-furan carboxylic esters,lauric esters, ω-undecylenic esters,and different amides.It has been found that the cercaricidal activity is influenced by the followings: (1)Substitution on the methyl carbon of the acetic esters with halogen,thiocyanogen or phenoxy group increases the activity. (2)The increase of the number of carbon atoms of the aliphatic acid part of the esters seems to be beneficial to the activity. (3)By increasing the substitution of halogen on the phenol ring,the activity is enhanced. (4)Changing of the amides to esters results in increasing activity. (5)By introducing diethylamino group into the β-position of the ethyl group of the ethyl esters,the activity is increased. The last one has the most pronounced effect,for example,B-diethylaminoethyl laurate is effective at 1:100,000,000 dilution,which is nearly 1,000 times more effective than the bis-(2- hydroxy-3,5,6-trichloro-phenyl)-methane(hexachlorophene).