Fruit flies (Drosophila melanogaster) were exposed to high-LET krypton (84Kr) ions at low (4.2 rad) and high (1,584 rad) doses and killed to assess acute (36 h post-exposure) and late (35 days post-exposure) effects in the brain by means of electron microscopy. The main findings were: (a) glycogen granules appeared in the neuroglial compartment 36 h after exposure to either dose and were no longer present in flies killed 35 days later, (b) neuronal alterations (swelling and membrane disruption) were observed 35 days after exposure to both doses, (c) changes in the neuroglia (electron-dense masses of concentrically arranged membranous structures) were seen 35 days after exposure. The data are discussed in relation to previous research in the fruit fly using argon (40Ar) charged particles and to other radiation studies performed in the mammalian brain with the view of using the insect brain as a model for detailed study of radiation effects on neurons, neuroglia, and the neuron-neuroglia unit.