ABSTRACT. Mirror mediated object discrimination was investigated in a captive female, humanreared, western lowland gorilla (Gorilla gorilla gorilla) approximately 26 years of age. The gorilla was trained to find a stimulus that was only visible by use of the mirror. The gorilla could not reach the goal object except by observing it in the mirror and her hand movement was not visible through the mirror. Further, it was hypothesized that this mirror mediated object discrimination would enhance the probability of self-directed behavior. GALLUP'S (1970) marking paradigm, excluding the use of general anesthesia, was utilized in assessing self-directed behavior. Mirror-gazing, face-directed, and mark-directed behavior with a mirror were compared before versus after discrimination mirror training. The results supported the hypotheses that gorillas are capable of mirror mediated object discrimination and that mirror training involving a discrimination task would facilitate self-directed behavior.