An oil spill near a breeding colony could result in the transfer of oil from the plumage and feet of incubating birds to their eggs. Microlitre amounts of No. 2 fuel oil were applied externally to common eider eggs in an island breeding colony in Maine. Clutches of eggs treated with 20 ?l of fuel oil had significantly greater embryonic mortality than the control clutches when they were examined 7 days after treatment. The results are similar to those of an earlier study of artificially incubated common eider eggs and indicate that nest site conditions do not affect embryotoxicity of No. 2 fuel oil.