The parasitoidDacnusa sibiricaTelenga is currently used for seasonal inoculative biological control of the leafminersLiriomyza bryoniae(Kaltenbach) andLiriomyza trifolii(Burgess) on glasshouse vegetables in northwest Europe. The braconid is effective from the beginning of the season until July, when average temperatures vary between 15 and 20°C. To estimate the potential reproduction (R0) of the parasitoid, the effect of temperature (15, 20, and 25°C) on its fecundity, longevity, and oviposition rate was examined in the laboratory. Fecundity and longevity decreased with increasing temperatures, but did not differ significantly between 20 and 25°C. Oviposition rate was highest at 20°C, apparently the optimum temperature for oviposition.The R0ofD. sibiricadecreased with increasing temperatures. Therefore,D. sibiricais expected to be less effective for the biological control of leafminers on tomato at high temperatures. The use of potential population growth, R0or rm, to evaluate parasitoids' effectiveness is discussed.