SUMMARY. 1. The conditions required to initiate development of resting eggs of thirty-six clones of Daphnia representing seven species were investigated.

2. The temperature of both dark incubation and subsequent light treatment are shown to affect hatch success. By varying these parameters the majority of resting eggs from each test clone were stimulated to develop. Arctic clones required a low hatching temperature (7°C), whereas clones from warmer climates hatched best at 14–21°C.

3. Variation in hatching cues existed between conspecific individuals from different collection sites. These differences suggest that research determining macro- and microgeographic patterns in hatching phenology would be fruitful.