Quantitative data on diet selectivity and numerical response of foxes (Pseudalopex spp.) are reported for two contrasting years in small mammal abundance, at Las Chinchillas National Reserve in north-central Chile. Relative density of foxes was assessed with scent station lines. Small mammal abundance was simultaneously evaluated using live traps. Fox diet was determined through analysis of scats collected during small-mammal trapping bouts. In both periods combined, 40 fox visits (22 in March 1988 and 18 visits in July 1990) were recorded for an effort of 105 operating stations/line-nights (38% success). Frequencies of fox visits between the two periods were not statistically different, in close agreement with the similar sighting records of foxes obtained for both years. During the first period, 619 small-mammal individuals were captured, in contrast to only 31 individuals during the latter. Consequently, foxes did not show the expected numerical response (local population decrease) when faced with a drastic population decline of their main prey. However, both during the prey abundance and scarceness periods, foxes showed marked preference (Octodon degus) or avoidance (Phyllotis darwini) despite the decrease of these prey. Foxes thus showed diet selectivity.