SUMMARY.1. The duration of egg incubation (Y) inDinocras cephalotesandSiphonoperla burmeisteriwas related to constant temperatures from 4 to 24°C, by the regression equations Y=2382T−1, 402(r2=0.992, P<0.001) and y= 2683T−1.667(r2=0.994,P<0.001), respectively. No diapause was observed in either species.2. Egg incubation inD. cephaloleswas slow and took 784.9±92.7 (mean ± SD) degree days between 12 and 20°C. significantly more than inS. burmeisteri(445±76.17 degree days:t= 7.44. d.f.=13,P<0.001).3. ForD. cephaloteshatching occurred at temperatures between 12 and 24°C, and for S. burmeisteribetween 8 and 20°C. The mean volume of the eggs ofD. cephaloteswas about 5 times greater than that ofS. burmeisteriand the mean body lengths of the newly‐hatched nymphs were 1.13 mm and 0.95 mm respectively.4. This study shows that the freshwater fauna of northern Fennoscan‐ dia also contains species with warm stenotherm eggs. D.cephalotes.which is of a Mediterranean origin (Zwick, 1981a), may exist at the limit of its distribution in norther