The application relates to a series of 2-imino-6-methylhexahydropyrimidin-4-one derivatives and 3-imino-5-methyl-1,2,4-thiadiazinane 1,1-dioxide derivatives of formula (I), substituted by an arylaminophenyl or heteroarylaminophenyl moiety. The compounds are potent inhibitors of the growth and propagation of the Plasmodium falciparum parasite in human blood and thus useful as pharmaceutical agents for the treatment of malaria.