A series of 2-imino-6-methyltetrahydropyrimidin-4(1H)-one derivatives, substituted in the 6-position by a phenyl moiety which in turn is meta-substituted by an optionally substituted unsaturated fused bicyclic ring system containing at least one nitrogen atom, being selective inhibitors of plasmepsin V activity, are beneficial as pharmaceutical agents, especially in the treatment of malaria.