Eggs from White Leghorn lines selected for fat and slow hatching time were incubated in continuous light and in unlighted incubators. The fast hatching line averaged 48 hours shorter in incubation time than the slow hatching line. The light treated eggs averaged five hours earlier in hatching time than the unlighted treatment. These two effects on hatching time, one genetic and one environmental, were completely additive. No effect of either treatment on hatchability was observed after all hatchable eggs had hatched.