The Texas horned lizard (Phrynosoma cornutum) is a threatened species in Texas, US (federal category C2). Causes for population declines in this species are likely multiple. Disease and health assessments using traditional methods, including hematology, have been minimally studied in Phrynosoma spp. Apparently healthy, free-ranging Texas horned lizard from four locations in Texas (n=53) and captive Texas horned lizards maintained at the Fort Worth Zoo (n=16) were sampled to establish hematologic reference intervals. Within the populations studied, total leukocyte, heterophil, and eosinophil counts were higher in managed compared to free-ranging lizards. Leukocyte and heterophil counts of all lizards were lower during the ovipositional months of June and July.