Abstract There are risks involved in the production and exportation of fruit fly hosts due to the possible spread of tephritid pests during distribution. Anastrepha grandis attacks cucurbit fruits and is considered an A1 quarantine pest in many countries. The objective of this study was to evaluate the effect of hot water treatment on the eggs and larvae of A. grandis in vitro, and on ‘Atlas' squash (Cucurbita moschata). The eggs and third-instar larvae of A. grandis were exposed to hot water at temperatures of 42.0, 44.0, 46.0, 46.5, 47.0, 47.5, 48.0, 49.0 and 50.0 (± 0.5) °C for durations of 0 (control), 10, 20, 30 and 60 minutes. Water temperatures of at least 44 °C affected the in vitro larval eclosion of A. grandis during all exposure times. No adults were obtained when in vitro A. grandis larvae were treated at 49 °C and 50 °C at all exposure times and, 48 °C for 30 and 60 minutes. No adults were obtained when squashes infested with A. grandis eggs or larvae were treated at temperatures of 49 °C and 50 °C during any exposure time, as well as subjected to 48 °C for 20 minutes. Anastrepha grandis larvae were slightly more susceptible to hydrothermal treatment than eggs in squashes. Hot water treatment applies at a temperature of 48 °C for 20 minutes is an effective phytosanitary treatment for squashes cv. Atlas infested with eggs and larvae of A. grandis.