The natural parasitism rate and effectiveness of the egg parasitoid, Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae), was studied for the first first time in Turkey in 2002 by rearing and releasing it against the cotton bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae). Releases of T. evanescens were made twice against the three generations of H. armigera during June, July, and August in the Cukurova region. Four replicates were used in both the Trichogramma release (without insecticides) treatments and the control (without Trichogramma and without insecticides). In each Trichogramma release, approximately 120,000 parasitoids ha⁻¹ (a total of 720,000 parasitoids ha⁻¹) were released. The parasitization rate on H. armigera averaged 52.5% in the release plots and only 28.5% in the control plots. Compared to the control plots, the reduction in the number of H. armigera larvae averaged 33.3% and the reduction in the number of damaged bolls averaged 42.8% in the release plots. Compared to the control plots, in fields where only insecticides were used, the number of H. armigera larvae was reduced by 68.0% and the number of damaged bollworm was reduced by 71.4%. In adjacent cotton fields treated with insecticides and excluded from the experimental area the natural parasitization rate of T. evanescens was low. Of the 241 H. armigera eggs of those collected on cotton, only 14 were parasitized (5.8%) by T. evanescens. Perhaps, pesticides used to control the cotton pests had non-target side effects on the parasitoids, and many T. evanescens could have been destroyed. Bollworm egg parasitism could be substantially increased with augmentative releases of T. evanescens wasps in cotton. Trichogramma evanescens has potential in the biological control of H. armigera.