NEW MATERIAL: Compounds of formula I [wherein R1 is bonded through C and is a (substituted) heteroaromatic group; X is O, S, SO, SO2, -CR5(CH)- (wherein R5 is H or an alkyl); R2 is H, a halogen, nitro, an alkyl or the like; and R3 and R4 are each H, a (substituted) alkyl, an alkenyl or the like], excluding compounds of formula I (wherein X is -CR5(CN)-; and R1 is thienyl).

EXAMPLE: 2-[4-(6'-Chloro)-2'-benzothiazolyloxy]-3,5-dichlorophenyl]- hexahydro-1,2,4-triazine-3,5-dione of formula II.

USE: Parasiticides against parasitic protozoa, Aramadillidium, fish parasites.

PREPARATION: A compound represented by formula III is hydrogenated, or a compound represented by formula IV is reacted with a compound of the formula: R3-B [wherein R3 is a (substituted) alkyl or the like; and B is a halogen, -O-SO-2-alkyl or the like].