Abstract: We investigated the effect of an essential fatty acid (EFA)-deficient diet on hibernation patterns in yellow-bellied marmots (Marmota flaviventris). Fatty acid (FA) analysis of white adipose tissue (WAT) from animals maintained for 2 mo on the EFA-deficient diet suggested that little or no EFAs were present in the gonadal or omental fat depots. Hibernation bout lengths of the EFA-deficient animals were significantly shorter (P < 0.01) than those of control animals. Stated another way, these animals aroused twice as frequently compared with control animals and used more energy to survive the winter. Analysis of WAT composition and blood samples revealed that animals were highly lipolytic during winter. Furthermore, the release of FAs was not random: linoleate (cis-9, cis-12 octadecadienoic acid, 18:2, a diene EFA) was significantly (P < 0.05) underrepresented in venous outflow from the gonadal fat pad based on percentage of this species in the WAT. The concentration of saturated FAs was higher than that predicted from the WAT-FA composition. We conclude that linoleate is preferentially retained within WAT and that concentrations of this EFA may influence hibernation behavior. Thus EFAs may have a thermoregulatory role in hibernation in addition to their role as essential precursors for physiologically important lipids after hibernation is over.