Piaget's theory of human cognitive development was applied to three gorillas at the Oklahoma City Zoo by operationalizing sensorimotor intelligence. The gorillas were of different ages, ranging from one to five years old. Previous research has shown that non-human primates complete Piaget's stages in the same order as humans, albeit not until later ages. These studies were conducted to determine how the cognitive development of non-human primates compares to that of humans, which is important in exploring how humans evolved their level of cognition. Two hypotheses were tested: 1) The gorillas will achieve Piaget's six substages of sensorimotor intelligence in the same order as humans; and 2) The gorillas will exhibit a slower rate of cognitive development as defined in Piaget's theory compared to humans. Observational behavioral data were collected via 30-m focal continuous samples for a total of 17 h, and their behaviors were categorized. Chi-square analysis revealed the gorillas generally completed the substages in the order predicted by Piaget's model (one-year-old gorilla: df = 5, x = 20, p = 0.00125; three-year-old gorilla: df = 5, x = 13.706, p = 0.018; five-year-old gorilla: df = 5, x = 11.444, p = 0.043). The gorillas also exhibited slower achievement of the substages, not achieving each substage until later ages compared to humans. Thus, the null hypothesis that there was no significant difference in the number of substages achieved by each gorilla was rejected.