FLIT-MLO and No. 2 fuel oil are sprayed on wetlands for mosquito control during spring and summer. In one experiment to assess the effects of the spraying on birds, mallard eggs were sprayed with amounts of No. 2 fuel oil equivalent to 2.34, 4.67, or 18.70 liters/ha or FLIT-MLO equivalent to 9.35, 46.75, or 140.25 liters/ha on Day 6 of incubation. In a second experiment, mallard eggs were sprayed with 9.35, 46.75, or 140.25 liters/ha of FLIT-MLO on Days 3, 6, 12, or 18 of incubation. Hatchability of eggs sprayed with the highest treatment level of each substance was significantly lower than that of controls for the first experiment. Hatchability of eggs sprayed with FLIT-MLO in the second experiment was never significantly lower than that of controls. Ducklings from the first experiment, 36-48 hr old, were cold stressed for 1 hr at 8 degrees C and then immediately tested for their ability to respond to a fright stimulus. Ducklings from the group of eggs sprayed with 140.25 liters/ha of FLIT-MLO ran a significantly shorter distance from the fright stimulus than did controls. The effects of the heaviest exposure to FLIT-MLO (140.25 liters/ha) on egg hatchability and behavior of newly hatched young are uncertain because of the contradictory results for hatching success in the two experiments. However, normal applications of FLIT-MLO (9.35-46.75 liters/ha) or No. 2 fuel oil (2.34-4.67 liters/ha) do not appear to pose a threat to the embryos of breeding birds.