Postdiapause larval development and adult emergence of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), were studied in three field populations collected at Amsterdam, Conrad, and Opheim, MT, at 10, 15, 20, 25, 30, and 35°C and at 43, 54–62, and 75–76% RH under laboratory conditions. No development beyond the larval stage occurred at either 10 or 35°C at any relative humidity. At 10°C, >75% of the larvae remained alive, but quiescent, whereas at 35°C, ≈50% of the larvae probably re-entered diapause. Temperature and population were the main factors affecting development time and adult emergence. The shortest development times occurred at temperatures of 20°C and higher, but development time increased sharply at 15°C. Adult emergence was highest at 20 and 25°C for the two populations collected at Amsterdam and Conrad (western Montana) and was highest at 15 and 20°C for the population collected at Opheim (eastern Montana). Emerged females were twice as heavy as males. Temperature and relative humidity did not affect weight, but emerged adults from the populations collected at Amsterdam and Conrad were heavier than adults emerged from the population collected at Opheim. Sex ratio of emerging adults was female biased and was strongly affected by population. The optimal conditions for larval–adult development of C. cinctus lie between 20 and 25°C and 60–75% RH. This data will be used to predict adult emergence and forecast the onset of the adult flight period in Montana wheat fields.