<p>Wildlife population die-offs are frequently driven by pathogen introductions. In El Palmar National Park (northeastern Argentina), crab-eating foxes (<i>Cerdocyon thous</i>) and Pampas foxes (<i>Lycalopex gymnocercus</i>) experienced increased mortality likely associated with canine distemper virus (CDV) during 2006–2007. In 2017, the incidental detection of a crab-eating fox with neurological signs of CDV-compatible paresis, together with an abrupt decline in fox abundance, raised concerns for a renewed outbreak. Questionnaire surveys of park staff and affiliated hunters documented recurrent discoveries of fox carcasses and foxes manifesting clinical signs characteristic of CDV. CDV RNA was later confirmed in a dying crab-eating fox with paresis using RT-PCR; the same individual tested negative for rabies by direct immunofluorescence, cell culture, and RT-PCR. Phylogenetic analysis of whole-genome sequences revealed that the 2018 fox CDV lineage clustered with contemporaneous lineage from domestic dogs in the region, and diverged from vaccinal lineages. Spotlight road counts indicated exponential growth of crab-eating foxes during 2006–2015 followed by collapse, whereas Pampas foxes remained stable at lower levels. Indices of relative abundance of both species declined sharply during 2017–2018 —concordantly across spotlight surveys, camera traps, and hunter sightings—, indicating a mass mortality event that transiently altered the fox species ratio. Population recovery was slow. Multiple, convergent lines of evidence implicate CDV —most plausibly of canine origin— as the proximate driver of the fox population collapse. These findings underscore the need to increase dog vaccination coverage and to promote responsible ownership within the surrounding watershed.</p>