Many animals exhibit pronounced shifts in ecology (e.g., habitat use, diet) as they grow. The central goal of this study was to determine whether habitat use and movement patterns of juvenile black ratsnakes (Elaphe obsoleta) differed from patterns previously documented for adult ratsnakes and to determine the conservation implications of any changes identified. We found a shift in habitat use by black ratsnakes with body size; juveniles used macrohabitats and microhabitats in proportion to their availability, unlike adult black ratsnakes that have been shown to prefer forest edges at both scales. Frequency of movement declined with body size, whereas distance traveled per move increased with body size. Habitat selection and movement patterns may be a result of ontogenetic shifts in thermoregulatory behaviour associated with changes in body size, or alternatively, may reflect size-related variation in predation risk. Home range size and fidelity, and fidelity to hibernacula all increased with body size. Despite ontogenetic changes in habitat use and movement by ratsnakes, recommendations regarding critical habitat for adults of this species should be adequate to protect juveniles.