Factors influencing egg mortality in Aphis gossypii hibernating on Hibiscus syriacus L. were examined. Observations on 380 hibernating eggs in the field showed that 42% hatched successfully next spring and that 87% and 6% of egg mortality were due to atrophy and falling from the branches, respectively. Egg atrophy occurred in the field from December to February. Oviparous females isolated without males in the laboratory often laid unfertilized eggs in early November. Fertilized eggs turned glossy black 7 days after oviposition, but unfertilized ones turned dark yellow or transparent except at the central part and atrophied 6 weeks later. Few of the eggs laid by mated oviparous females died of atrophy within 6 weeks after oviposition. These results suggest that egg atrophy in the field is caused by a considerable number of unfertilized eggs in hibernating eggs.