Food selection by adult red foxes Vulpes vulpes was studied in a mountainous environment characterised by fluctuating populations of water voles. According to optimal foraging theory, the diets of adults and cubs should be similar during vole highs, with voles as staple prey. On the other hand, a difference should occur when water voles become scarce. Adults should then bring larger alternative prey to their cubs, and consume smaller prey at the capture site. Adult and cub scats were collected around breeding dens during a 6‐year period, from the peak to a near‐extinction phase of the water vole population, and analysed. As predicted by the optimal foraging theory, no significant difference was found between the diets of cubs and adults when voles were abundant. Furthermore, cubs consumed a higher proportion of large alternative prey, i.e. lagomorphs, than adults in the first year of low vole density. Unexpectedly, this difference did not persist the following years despite the water vole scarcity. The consumption of large alternative prey at the den decreased significantly, whereas the consumption of small alternative prey, i.e. invertebrates, increased. Consequently, the composition of prey brought to the den was similar to that consumed at the capture site. Food selection was probably affected by the unpredictability of large prey availability and accessibility. It is suggested that if small alternative prey proves less profitable to the cubs than large prey, a long‐lasting shortage of water voles could possibly become a factor regulating fox numbers in the area studied.