To investigate the mechanisms by which the isopod Limnoria tolerates copper, chromium and arsenic when tunnelling CCA-treated wood in the marine environment, digestive caecae of specimens of L. quadripunctata from CCA-treated and untreated Pinus sylvestris sapwood were examined using transmission electron microscopy (TEM) and energy dispersive X-ray detection (EDX). Copper-containing granules were found to be present in the small cells of the digestive caecae (midgut) of Limnoria from both treated and untreated wood. However, individuals tunnelling treated timber contained a greater number of granules, with six types of morphology being distinguished of which only two were present in Limnoria from untreated wood. The presence of copper-containing granules in Limnoria from untreated timber may be explained by the ability of this isopod to acquire and store copper from seawater for use in enzymes and blood pigments. The increased number of copper granules in individuals tunnelling treated wood was believed to result from elevated levels of this element in the food substrate. This ability to store copper allows these organisms to tolerate this metal in the wood substrate and may explain how this organism is able to attack CCA-treated wood in service. No chromium, or arsenic, was detected in the digestive caecal cells.