The hatching ofNezara viridulawas found to be a function of a photoperiodically entrained rhythm. Periodicity of hatching found in a constant photoperiod was shifted by 12 h from that found in a 14:10 (L:D) photoperiod and the former was probably a reflection of periodicity of oviposition. Both the onset and distribution of the photoperiodic hatching rhythm were dependent on the temperature of incubation. Although emergence was temperature dependent, it was not thermoperiodically entrained. The synchronization of hatching within an egg mass was the result of the interaction of emerging nymphs stimulating hatching in adjacent eggs and was not due to independent timing of eclosion of each egg. Thus, in addition to timing eclosion at the onset of light, the photoperiodic rhythm may also have functioned to synchronize development so that all eggs in an egg mass were simultaneously capable of hatching when stimulated by the emergence of siblings.