1. Four experiments involving 23,800 settable eggs of commercial broiler parent stocks were conducted to determine the effect of gamma irradiation before egg incubation on hatchability and body weight of hatched chicks. 2. Irradiation doses were 0, 0.05, 0.10, 0.15, 0.20, 0.40, 0.80 and 1.2 Grays (Gy) for experiment 1; 0, 0.15 and 1.2 Gy for experiment 2; 0 and 0.15 Gy for experiment 3; 0, 1.6 and 2.1 Gy for experiment 4. Dose rate was about 0.12 Gy/min. 3. Gamma irradiation at doses of 0.05 to 1.6 Gy before incubation did not affect embryo development, whereas a dose of 2.1 Gy was harmful.