The life-history of Aeshna viridis Eversm. has been studied at a peat-pit in southern Sweden by means of regular sampling. Larvae from some samples from late summer to early spring have been experimentally subjected to constant artificial day-lengths of I3, I4.5, I6 and I9.5 hours at a temperature of 20°C. The results of experiments suggest that the rapid transition of photoperiod in autumn and spring is most important in the regulation of the life-cycle, which in the population studied had a duration of 2-3 years. A comparison is made between the seasonal regulation in Aeshna viridis and that in some other species. The causes of the phenological differences between certain types of spring and summer species and the differences in life-history of Aeshna species at different latitudes are discussed.