Geographic variation in body size of Sylvilagus floridanus was investigated from 3 climatically different and disjunct regions of the range of this species (region 1, northeastern United States and southern Canada; region 2, southeastern United States; region 3, southwestern United States and northern Mexico). On a rangewide scale, body size decreased from east to west and less strongly north to south, consistent with Bergmann's rule, and was associated most strongly with variability in winter precipitation. On a regional scale, body size decreased from northwest to southeast in region 1 and southwest to northeast in region 3. We detected no trend in region 2. Large body size in region 1 was found in cold and dry environments. Body size in region 3 increased with cooler and wetter summer environments, high elevations, and low variability in winter precipitation. Bergmann's rule was supported within region 1 but not in region 2 or 3. Directions and magnitudes of trends in body size appear to depend on severity of climate; the 2 most climatically stressful regions (1 and 3) had stronger climatic correlations than region 2.