Energy flow pathways of the red fox (Vulpes fulva) were measured and evaluated under penned conditions. Four fox pups, each 13-14 weeks old, were housed singly in animal runs for 10 weeks beginning 1 July 1971. Domestic rabbits served as the energy source for the entire study. Least squares analysis provided a regression line equation (R = 0.98) of Y = 0.16X + 0.57 with Y being live weight and X being age in weeks for first of the year pups. It was estimated that pups had reached full growth by midto late-September. Female weights consistently averaged less than male weights. Mean energy flow determinations for red fox pups, expressed as kcal/kg body wt/day ? SD, were as follows: ingestion 223 ? 71, assimilation 203 ? 63, respiration 193 56, feces 20 -+9, and secondary production 10 ? 9. Mean assimilation efficiency (assimilation/ingestion x 100) was 91 percent. Ecological (production/ingestion x 100) and tissue (production/assimilation X 100) growth efficiency were approximately 4 percent each. Based on the energetics of the present study and field data from related studies, it was estimated that a red fox population would utilize only 3 percent of its rabbit population