AbstractAdults of the autumn-emerging capniid stonefiy, Mesocapnia oenone (Neave), were collected from the Bow River in the Canadian Rocky Mountains. After oviposition their eggs were incubated in the laboratory. Successful hatching occurred in the range 2°–15 °C; higher temperatures, except for a short period, were lethal. Eggs subjected to sub-zero temperatures, up to 17 days at −12 °C, also hatched successfully. Within the range 2°–15 °C there was a relationship, linear on logarithmic scales, between water temperature (T °C) and the egg incubation period (Y days), expressed by the equation: Y = 197T−0.59 (r2 = 0.99, P < 0.001). The number of degree-days required for hatching (Z degree-days) increased with increasing temperature (T °C) and also showed a relationship, linear on logarithmic scales, expressed by the equation: Z = 197T0.41 (r2 = 0.97, P < 0.001). In contrast to most other Capniidae, the winter is spent in the egg stage in M. oenone. Our data demonstrate that a combination of a low number of degree-days required for hatching at low temperatures and reduced sensitivity to higher temperatures ensures that egg hatching occurs in the spring. The characteristics of egg development in the autumn-emerging M. oenone are compared with the spring-emerging Capnia atra.