Walnuts are a valuable crop the sale and export potential of which may be severely limited by contamination with aflatoxins, metabolites produced on infection with Aspergillus flavus. The effect of a series of four naphthoquinones [1,4-naphthoquinone (1); juglone (5-hydroxy-1,4-naphthoquinone) (2); 2-methyl-1, 4-naphthoquinone (3); and, plumbagin (5-hydroxy-2-methyl-1, 4-naphthoquinone) (4)] (Figure 1), which occur in walnut husks, on fungal viability and aflatoxigenesis was studied in vitro. The quinones delayed germination of the fungus and were capable of completely inhibiting growth at higher concentrations. Their effect on aflatoxin levels was highly dependent on the concentration of individual naphthoquinones in the media. At higher concentrations, aflatoxin production was decreased or completely inhibited, but at lower concentrations there was a stimulatory effect on aflatoxin biosynthesis, with a >3-fold increase at 20 ppm of 3. Structural features associated with decreased fungal viability and greatest effect on aflatoxigenesis are the presence of a 5-hydroxyl or 2-methyl substituent, but there is no significant additive effect when both of these substituents are present.