Insects, like vertebrates, express iron regulatory proteins (IRPs) that may regulate proteins in cellular iron storage and energy metabolism. Two mRNAs, an unspliced form of ferritin H mRNA and succinate dehydrogenase subunit b (SDHb) mRNA, are known to comprise an iron responsive element (IRE) in their 5'-untranslated region making them susceptible to translational repression by IRPs at low iron levels. We have investigated the effect of wild-type human IRP1 (hIRP1) and the constitutively active mutant hIRP1-S437 in transgenic Drosophila melanogaster. Endogenous Drosophila IRE-binding activity was readily detected in gel retardation assays. However, translational repression assessed by polysome gradients was only visible for unspliced IRE-containing ferritin H mRNA, but not for SDHb mRNA. Upon expression of exogenous hIRP1-S437 both mRNAs were strongly repressed. This correlated with a diminished survival rate of adult flies with hIRP1 and complete lethality with hIRP1-S437. We conclude that constitutive IRP1 expression is deleterious to fly survival, probably due to the essential function of SDHb or proteins encoded by yet unidentified target mRNAs.