Influence of temperature on the development and hatching of eggs ofSimulium arcticumMalloch IIS-10.11 was determined. Temperature-dependent development and hatching at six constant temperatures are described. The rate of egg development was greatest at 18°C. The data fitted a six-parameter development model. Linear regression of the development rate versus temperature provided estimates of base temperature and development time in degree-days for 50 and 90% hatching of eggs. Results are compared with reports on egg hatching and larval distribution ofS. arcticumsensu lato in Alberta, Canada.