Fungicides inhibiting the biosynthesis of ergostrol, such as the triazoles and imidazoles, have been shown to enhance the effect of insecticides on birds, mammals and invertebrates in the terrestrial environment. The synergy is proposed to be due to an effect on P450 monooxygenase enzymes active in pesticide metabolism in these organisms. Fungicides often enter the aquatic environment jointly with other pesticides. It is therefore possible that they could act as synergists also in the aquatic environment. In this study we tested the joint effect of the imidazole fungicide prochloraz together with the herbicides acifluorfen, diquat and terbuthylazine, the fungicide azoxystrobin and the insecticides chlorfenvinphos, dimethoate, and pirimicarb on the bacteria Vibrio fischeri (six binary mixtures), the crustacean Daphnia magna (four binary mixtures), the algae Pseudokirchneriella subcapitata (four binary mixtures) and the floating plant Lemna minor (three binary mixtures). All the binary mixtures were evaluated both in relation to the model of concentration addition (CA) and independent action (IA) using isobolograms. The study showed strong synergy in relation to CA between prochloraz and azoxystrobin, diquat and esfenvalerat on D. magna with sums of toxic units for the 50:50% effect mixture (∑TU_50:50) as low as 0.25. The mixture with dimethoate was however antagonistic with ∑TU_50:50 of 2.04. Four out of the six mixtures testes on V. fisheri showed synergy in relation to CA, but for three of the mixtures the response could be explained by IA. Only the mixture with diquat showed synergy in relation to both IA and CA with ∑TU_50:50 around 0.50. There was no significant synergy for any of the combinations tested on the plant and the algae species in relation to CA and only for diquat in the algae-test in relation to IA. Hence, prochloraz does synergise the effect of some pesticides in the aquatic environment, but not consistently across species. The organism most susceptible to synergy by prochloraz in this study was D. magna. Especially the combination with insecticides such as esfenvalerate, where the concentration needed to immobilize 50% of the daphnia was reduced from >3 μg L⁻¹ to less than 0.5 μg L⁻¹ when prochloraz was added, could be problematic as these concentrations are environmentally realistic. Furthermore, insecticides and ergostrol-biosynthesis-inhibitors (EBI-fungicides) are often applied together, and are therefore likely to co-exist in surface waters, enhancing the problem of the already very potent insecticides.