The response of eukaryotic cells to ionising radiation (IR)-induced double-strand DNA breaks is highly conserved and involves a DNA repair mechanism characterised by the early phosphorylation of histone protein H2AX (pro-ducing the active form γH2AX). Although the expression of an induced γH2AX variant has been detected in Drosophila melanogaster, the expression and radiation response of a γH2AX homologue has not been reported in economically important fruit flies. We use Bactrocera tryoni (Diptera: Tephritidae, Queensland fruit fly or 'Q-fly') to investigate this response with a view to developing molecular assays to detect/quantify exposure of fruit flies to IR and conse-quent DNA damage. Deep sequencing confirmed the pres-ence of a H2AX homologue that we have termed H2AvB (i.e. variant Bactrocera) and has an identical sequence to a histone reported from the human disease vector Glossina morsitans. A linear dose–response of γH2AvB (0–400 Gy IR) was observed in whole Q-fly pupal lysates 24 h post-IR and was detected at doses as low as 20 Gy. γH2AvB signal peaked at ~20 min after IR exposure and at 24 h post-IR the signal remained elevated but declined significantly by 5days. Persistent and dose-dependent γH2AvB signal could be detected and quantified either by western blot or by laser scanning cytometry up to 17days post-IR exposure in histone extracts or isolated nuclei from adult Q-flies (irra-diated as pupae). We conclude that IR exposure in Q-fly leads to persistent γH2AvB signals (over a period of days) that can easily be detected by western blot or quantitative immunofluorescence techniques. These approaches have potential as the basis for assays for detection and quantifi-cation of prior IR exposure in pest fruit flies.