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The synergistic interaction between ergosterol biosynthesis inhibiting (EBI) fungicides and a pyrethroid insecticide was studied in the honeybee, Apis mellifera L. The investigation results from reports by farmers that tank-mixing of these particular pesticides was causing a higher than expected honeybee mortality. The aim of the study was to demonstrate and quantify the synergistic effect with laboratory and field experiments, and to elucidate the underlying biochemical mechanism causing the enhanced toxicity. In the laboratory, the pyrethroid insecticide lamba-cyhalothrin was combined with a range of EBI fungicides, at ratios according to their recommended application rates, and dosed typically onto the thorax of honeybees. Mortality assessments 24h after dosing were used to generate LD₅₀ values and synergistic ratios. All the fungicides tested increased the toxicity of the pyrethroid to bees. The fungicide propiconazole was found to have the strongest synergistic effect, decreasing the LD₅₀ of lambda-cyhalothrin from 68.0ng ai bee⁻¹ to 4.2ng, thus having a synergistic ratio of 16.2. Cage studies showed that honeybees were susceptible to residues of a mixture of the fungicide prochloraz and lambda-cyhalothrin. Consequently, semi-field experiments were conducted to determine the hazard of tank-mixing lambda-cyhalothrin with prochloraz to honeybees foraging on simulated aphid honeydew on winter wheat. Results indicated a large honeybee mortality directly after spraying the tank-mixed pesticides, however the repellency or reduced foraging effect of pyrethroids reduced subsequent exposure of foragers to the pesticide mix, and thus lowered the hazard. The mechanism by which the fungicide prochloraz enhances the toxicity of lambda-cyhalothrin was investigated. In vitro incubations with honeybee midguts were used to study the metabolism of ¹⁴Cλ-cyhalothrin alone, and in combination with prochloraz. Pre-incubation with the fungicide revealed an inhibition of microsomal monooxygenase activity.