Eggs of E. cynosura were cultured under natural and controlled conditions to determine both the diel periodicity in hatching and whether photoperiod and fluctuating temperature act as environmental cues. 150 egg masses were examined under 10 environmental conditions: 3 pretreatment photoperiods, 3 posttreatment photoperiods, 2 temperature regimes, and a natural control. Hatchlings were enumerated and removed from the cultures 6 times each day. The data were transformed to percent hatch for 2- and 3-factor analyses of variance. A diel periodicity under both the natural and 12L:12D photoperiods was evidenced by hatching peaks (80-86 percent of the hatch) immediately after the onset of darkness. Under constant light and constant darkness no peak was obtained because of the large variations in hatching throughout the day. Eggs transferred to constant light or constant dark after 8 days of development under 12L:12D (eye spots present) exhibited no peak. This is evidence that the diel periodicity is not endogenously controlled. Exogenous control is evidenced by the difference in time of hatching peaks between natural and 12L:12D photoperiods (that is, a peak between 1800 and 1815 CDT for 12L: 12D vs. a peak between 2045 and 2100 CDT for natural photoperiod). These times encompass the onset of darkness which triggered an immediate hatching response. Temperature fluctuation does not appear to cue hatching.