Effects of relative humidity at 23°C on eggs of the bluegrass webworm from a New Mexico and a Kentucky population were compared. Over 90% of the fertilized eggs hatched above 60% RH in both populations. Although mortality varied inversely with RH between 10 and 50%, eggs from the New Mexico population exhibited comparatively greater hatching success and therefore appear to be more resistant to desiccation. Completely dry air was lethal to all eggs of both populations.Development of bluegrass webworm eggs from New Mexico accelerated as relative humidity increased. At 30% RH, the required time for half of the eggs to hatch was 228–240 hr; at 100% RH the required time was 180–192 hr.No significant differences between the 2 populations were found in terms of how maternal decapitation affected hatching and fertility of eggs.