Body mass – gut fill scaling relationships affect rate of digestion, foraging behavior, niche differentiation, and trophic interactions. On an intraspecific level, the scalar of this relationship has been reported to be both iso- and allometric (<1.0). We hypothesized the scalar of rumen-reticulum fill depends on diet. When the diet has low concentrations of indigestible fiber the scalar should be allometric to fulfill metabolic demands and isometric when the diet has high concentrations of indigestible fiber as both small and large animals have difficulties processing forage. We collected 50 male and 50 female white-tailed deer (Odocoileus virginianus) over a 30 h period from a site in south Texas in Oct. Animals ranged in body mass from 32 to 106 kg. We also recorded collection time, sex, lactation status of females, back fat thickness, and tooth wear. These covariates affected the body mass – rumen-reticulum fill relationship. Acid detergent fiber, our surrogate of indigestible fiber, in the rumen contents of our study animals was lower in concentration than in another study of white-tailed deer that estimated an isometric scalar for rumen-reticulum fill. The estimated scalar between body mass and rumen-reticulum fill in our study was 0.74 and allometric. Rumen-reticulum fill probably was influenced by chemostatic feedback and mechanisms were governed by metabolic demands. Rumen-reticulum fill scalars for white-tailed deer are not static but dynamic.