The antifungal activity of imazalil against spores of Penicillium italicum and several other fungi was found to be pH-dependent. The fungicide was much less effective at pH 5.2 than at pH 7.0. However, related members of the imidazole class of fungicides (miconazole and clotrimazole) did not show this pH dependency. Fungitoxicity of imazalil and miconazole at different pH’s was correlated with the uptake of the fungicides by P. italicum. Both fungicides were rapidly taken up at pH 7.0 but only miconazole was taken up in appreciable amounts at pH 5.0. Several compounds (tocopherol, farnesol and trilinolein) were antagonistic to the action of the imidazole fungicides. The rate of partitioning of imazalil, which has the same pKa as miconazole, into tocopherol was pH-dependent. However, miconazole partitioned into the antagonist independent of pH. Since pH also had no effect on fungitoxicity of miconazole it would suggest that substitution of the olefenic group of imazalil with a dichlorophenyl group enhances partitioning and uptake of the fungicide regardless of the state (protonation) of the nitrogen group of the imidazole moiety. Spores of P. italicum treated with a minimum inhibitory concentration of imazalil showed a reduced germination rate, swelling of both germinated and ungerminated spores, distorted germ tubes and loss of cytoplasm from germinating conidia. Inhibition of growth of P. italicum in liquid culture occurred within 10 hours after the addition of minimum inhibitory concentrations of the fungicide.