When holometabolous insect larvae are exposed to a radiation treatment, morbidity or mortality are typically manifested during a major developmental transition, usually a transition involving ecdysis. Thus, early instars fail to develop into later instars or the later instars fail to pupate or pupariate. Over a range of sublethal doses of gamma radiation (increments of 0, 15, 20, 25, and 30 Gy) applied to third-instar Mexican fruit flies, Anastrepha ludens (Loew) (Diptera: Tephritidae), infesting or implanted in grapefruits, Citrus paradisi Macfayden, survival decreased with increasing dose. At all radiation doses, the majority of treated larvae arrested development at pupal ecdysis, the transformation from a cryptocephalic to a phanerocephalic pupa. More than 96% of treated larvae died at, or before, reaching this transition at the highest dose tested (30 Gy). Contrary to expectations, the radiation treatment did not cause atrophy of the imaginai tissues, a result that we attribute to apoptosis.