We examined clinal variation in body size in the Texas Horned Lizard, Phrynosoma cornutum. We used principle component analysis of 10 morphological characters taken from 230 horned lizards from localities between 100°W and 105°W longitude, from central Mexico to southeastern Colorado (23.75°N to 38.09°N latitude). Females (N = 114) and males (N = 116) were treated separately. The first principle component (size) accounted for 68.4% of the variation in males, with a significant decrease in PC1 score with increased latitude (P < 0.0001, R² = 0.243). In females, the first principle component accounted for 71.9% of the variation, with a significant decrease in PC1 score with increased latitude (P < 0.0001, R² = 0.306). There was no significant variation in PC1 with longitude (P = 0.388).