Trissolcus grandis is an important egg parasitoid of sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae), the most serious pest of wheat in Iran. The thermal requirements of two populations of T. grandis were studied at five constant temperatures ranging from 20-32°C. Thermal thresholds for development were calculated using linear regression and degree-day models were determined by fitting non-linear equations to the data. The lower threshold for development was estimated to be 12.5 and 12.1°C, respectively, for males and females of the Bonab population, compared to 14.4 and 14.5°C for those of the Qaramalek population. Complete development required 143.8 and 162.8 degree-days, respectively, for males and females of the Bonab population and 116.9 and 124.6 for those of the Qaramalek population. Thus, wasps from the warmer region (Qaramalek) developed faster than those from the cooler region (Bonab), but had a higher thermal threshold for initiating development. Bonab females attained their highest fecundity (117.7 ± 7.2) at the lowest temperature tested (20°C), whereas the fecundity of Qaramalek females was maximal (96.8 ± 11.5) at 26°C. Biological control programmes that seek to augment wasp populations in wheat fields early in the spring, when natural rates of sunn pest parasitism tend to be low, should consider wasp thermal requirements to ensure the selection and release of locally-adapted parasitoids.