Llamas (Lama glama) are frequently used as guard animals by sheep producers as part of their predation management programs. However, few data are available concerning physical and behavioral attributes that distinguish between effective and ineffective guardian llamas. Our study addressed this issue and evaluated aggressiveness of llamas toward dogs. Initially, we identified physical and behavioral traits of individual llamas. Twenty llamas were randomly assigned to one of four groups (n = 5 per group). We used focal group sampling techniques to rank individual llamas according to frequencies with which they displayed alertness, leadership, dominant, aggressive, and threatening behaviors as well as postures indicating dominance or subordination. We then examined the behavior of individual llamas with sheep. Finally, we documented interactions among llamas, sheep, and a surrogate predator (border collie), our results showed that leadership, alertness, and weight of llamas were correlated with aggressiveness displayed toward the dog (p(s) = 0.064, 0.012, and 0.039 respectively). These traits are easily recognized and can be used by producers to select llamas as livestock guardians.