Larval growth of E. danica Mueller in a South Swedish stream was calculated from changes in the size distribution of the larvae between monthly samples. Part of the population completed a generation in 2 yr; the rest of the individuals spent 3 yr as immatures. Adult and larval sex ratios equalled 3:2 with an excess of males. Stepwise development of external male characters accompanied by decreasing relative energy content of the nymphs took place in 2 periods (late spring and autumn). Such a development of adult characters provides a mechanism allowing nymphs of different age to mature and emerge synchronously.