A mean dose of 249.58 Grays (Gy) 60Co irradiation caused greater than 99.9968% mortality of 99 817 irradiated Mexican fruit fly, Anastrepha ludens (Loew) eggs and larvae which infested ‘Ruby Red’ (also called ‘Webb Red Blush’) grapefruit (Citrus paradisi MacFayden). Determination was based on estimated larval populations (number of pupae in control fruit) and pupal mortalities (number of adults that eclose). Both 137Cs and 60Co required >15% dose of irradiation to kill 99.9968% of 21-25 days-old larvae in the fruit compared to the dose required to kill eggs and larvae up to 17 days old. The LD50s of eggs and larvae up to 17 days of age and 21-25 days-old larvae were statistically similar based on estimated larval and pupal mortalities. The LD50s were 93 and 82% less for early stage and mature larvae, respectively, than doses required for 99.9968% mortality.