A series of 3-imino-5-methyl-1,2,4-thiadiazinane 1,1-dioxide derivatives of formula (I), substituted in the 5-position by a phenyl moiety NH which in turn is meta-substituted by an optionally substituted fused bicyclic heteroaromatic 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.