We studied the effects of temperature on growth, consumption, assimilation, conversion efficiencies, and molt ofNematus calaisKirby (Hymenoptera: Tenthredinidae), a willow sawfly, in interior Alaska. Growth rate of fourth instars increased across the full range of naturally encountered temperatures (biomass doubling time of 6.1 d at 6°C versus 0.85 d at 30°C). Growth rate of fifth instars also increased from 6 to 24°C, but had generally lowerQ10Sthan fourth instars, and actually decreased from 24 to 30°C. Patterns in growth rate across temperatures were almost entirely a function of patterns in consumption rate; relative consumption rate increased sixfold from 6 to 24°C, whereas conversion efficiencies were constant across temperatures (approximate digestibility = 0.41 to 0.45, efficiency of converting digested matter = 0.24 to 0.31). The duration of the fourth molt decreased from 1.32 d at 6°C to 0.17 d at 24°C and then increased to 0.40 d at 30°C. Compared with more southerly distributed insects, growth rates ofN. calaistended to be less temperature sensitive and to peak at lower temperatures. Similarities betweenN. calaisandDineura virididorsata(Retzius), a sawfly from northern Europe, suggest that the Tenthredinidae may be fundamentally different in their molting physiology from the Lepidoptera and Coleoptera; the ability of tenthredinids to molt rapidly at low temperatures may contribute to their ecological success in high-latitude environments.