Four inhibitors of the lysine biosynthetic enzyme dihydrodipicolinate synthase (DHDPS) were examined for their effects on the late blight fungus Phytophthora infestans. One of these compounds, dipicolinic acid, reduced mycelial growth of P. infestans by 61% and completely controlled blight infection of leaf discs when used at 1 mM. The other inhibitors, chelidamic acid, chelidonic acid and dimethyl cis-piperidine-2,6-dicarboxylate, were less effective against P. infestans in vitro and in vivo. The effects of dipicolinic acid and chelidonic acid on mycelial growth were partly overcome by the addition of lysine to the growth medium. Both of these compounds reduced DHDPS activity and lysine concentrations in P. infestans, although they had no effect on activities of aspartate kinase and diaminopimelic acid decarboxylase. Both dipicolinic acid and chelidonic acid reduced in vitro activity of DHDPS from P. infestans.