The large mass of a turtle's shell presumably reflects a trade-off between its protective function and its imposition on the energy cost of locomotion. To examine this, we developed a relationship between body mass and shell linear dimensions in Ornate Box Turtles, Terrapene ornata, from north-central Nebraska. Using the relationship between body mass and plastron width, we estimated body mass of box turtles killed on roadways, and we cleaned and weighed their shells. Shell mass may be a linear function of body mass in these turtles, amounting to approximately a constant 30% of their body mass. Alternatively, shell mass may be an exponential function of body mass, increasing with body mass to approximately the 1.2 power; in this case, shell mass increases more quickly than body mass in these animals. We are unable to distinguish between these models based on our analysis, but the increase in shell mass with body mass is more consistent with the shell serving a supportive, rather than a protective, function. Plastron thickness increases with body mass to approximately the 0.47 power in our animals, which is also higher than expected if the shell serves solely a protective function.