At laying, eggs of Aedes aegypti (L.) weighed about 5 μg. As the embryo developed and the eggs became conditioned, weight increased to 12 � .03 μg. Conditioned eggs subjected to various relative humidities showed a progressive loss in weight and a corresponding decrease in hatchability. “Optimally” conditioned eggs held at 0% relative humidity at 25� � 2� C. decreased in weight to near 3.5 μg in 72 clays at which time hatching ceased. Suboptimally conditioned eggs of the same initial weight decreased to the same extent in about 55, 80, 150, 205, and 230 clays in relative humidities of approximately 0%, 30%, 60%, 75%, and 90% respectively. A high correlation between the changes in weight and hatchability is demonstrated.