The diurnal periodicity of emergence of several stonefly and mayfly species was studied on the shore of lake Mondsee, Austria. Nemoura cinerea (Plecoptera) emerges during the whole day with peaks in the morning and afternoon. The Ephemeroptera Leptophlebia marginata, Ephemera danica and Centroptilum luteolum emerge as subimagines at noon and in the afternoon, Ephemerella ignita in the evening around sunset. The duration of the subimaginal life of these mayfly species was determined. The importance of the temperature-dependent length of subimaginal life for the diurnal periodicity of the imagoes' emergence from subimaginal stage is discussed.