The effects of external exposure of mallard (Anas platyrhynchos) eggs to malathion, diazinon, and parathion were examined using formulations and concentrations similar to field applications. Treatment with aqueous emulsion simulated exposure at the rate of 100 gal per acre (153 liters/hectare) with three to six different doses per compound with treatment at 3 and 8 days of embryonic development. Treatment with a nontoxic oil vehicle simulated exposure at the rate of 11 gal per acre (16.8 liters/hectare) with three to six different doses per compound. The order of embryotoxicity on a pounds-per-acre basis was parathion > diazinon > malathion with either vehicle. However, the potential hazard under conditions of up to five times the maximum field level of application was greater for malathion because of the high permissible level of application for malathion on certain crops. Parathion, the most embryotoxic of the three, had the most pronounced effects when an oil vehicle was used, as reflected by an LC₅₀ of about 2 lb of active ingredient per acre, stunted growth, and a high frequency of malformations involving distortion of the axial skeleton, particularly in the cervical region. All three compounds resulted in significant depression of plasma and brain cholinesterase activity, but parathion caused the most depression throughout development, which was still apparent in hatchlings. Treatment with either distilled water or oil vehicle alone did not result in any of these effects seen with organophosphorous insecticides.