The fungal metabolite kojic acid, which is produced by Aspergillus and Penicillium species fungi that may be pathogens of both insects and plants, was a significant inhibitor of phenoloxidase of different representative beetle and caterpillar insect species. Fusaric acid and picolinic acid, produced by Fusarium spp., were also significant inhibitors of phenoloxidase, while dipicolinic acid and beauvericin were ineffective at concentrations tested. Previous reports of the ability of kojic and fusaric acid to inhibit defensive enzymes of plants suggest that these compounds may be important in allowing the producing fungi to be pathogens of both insects and plants.