Reliable and abundant resources are likely to favour specialization, while unpredictable environmental variation should favour a generalist strategy. The rodent population cycles of northern latitudes can be seen as both predictable and unpredictable, depending on the scale in time and space. The arctic fox Alopex lagopus is an opportunistic carnivore, but paradoxically, it seems to function as a specialist on fluctuating rodent Arvicolinae populations in most inland areas. We have studied the dietary response of arctic foxes in Sweden during 5 years of varying abundance of Norwegian lemming Lemmus lemmus, and how these changes influenced the reproductive success of the foxes. The arctic fox population on mainland Fennoscandia is threatened by extinction and the situation has deteriorated during the 1980s and 1990s because of an absence of lemming peaks. Our results showed that in all years, lemming was the main prey for arctic foxes, with 85% frequency of occurrence in summer faeces (scats). Bird remains (mainly Passeriformes) were present in 34% of the scats, reindeer Rangifer tarandus in 21%, voles and shrews in 4% and hares Lepus timidus in 2% of the scats. The occurrences of lemming, bird and larger mammal (reindeer and hare) remains in the scats varied significantly between years. Temporal variations within summer seasons and dietary differences between sub-areas, indicated that arctic foxes fed opportunistically on the alternative prey types. Den occupancy rates were positively correlated with lemming population densities during the previous winter, indicating a strong numerical response. We conclude that from a functional aspect, the arctic fox in Sweden is a lemming specialist, since lemming is the main prey and their abundance is the best predictor of arctic fox reproductive success. Other prey are used opportunistically in relation to their availability.