The round-tailed horned lizard Phrynosoma modestum was active and feeding throughout the day from 2 hr after sunrise until sunset. Phrynosoma modestum remained in the shade of large mesquite during midday where they preyed on honey pot ants (Myrmecocystus depilis/mimicus) which were foraging in the canopy and moving across the shaded substrate. Phrynosoma modestum fed at an average of 3.9 +- 2.2 ants bout-1 and average of 7.0 +- 1.7 bouts hr-1. The species diversity of the diet of P. modestum doubled (H' = 0.78 to H' = 1.41) following rainfall when many species of small ants extended their activity to overlap with the activity of P. modestum. Honey pot ants Myrmecocystus mimicus/depilis were, however, the only dependable prey. Simulated predation of 20 ants day-1 and 40 ants day-1 affected activity of M. mimicus/depilis , while simulated predation of 10 ants day-1 had no effect. We estimate that P. modestum removes approximately one-half of the early summer standing crop of M. mimicus/depilis during the summer.