Captive breeding programs are utilized worldwide for the recovery and conservation of endangered species, including the critically-endangered hawksbill turtle (Eretmochelys imbricata). However, aggression is an issue that is commonly reported among captive hawksbill turtles housed together. In the present study, we installed rocks in a captive environment to assess whether they can suppress aggressive behavior in captive hawksbill turtles. The behavior of immature female hawksbill turtles (n=6) was recorded over a 10 d period (between 0900 and 1700 h daily) in an experimental tank under the following conditions: (1) housed alone or paired with another individual in the (2) presence and (3) absence of rocks. Resting behavior was recorded as the total amount of time spent resting per day, while hiding and biting behaviors (i.e., aggressive behavior) were recorded as the total number of events observed per day. Serum corticosterone concentrations were measured at 0900 and 1700 h daily throughout the experimental period. The median number of hiding and biting events was significantly lower for paired individuals housed in the presence of rocks than that for those housed in the absence of rocks. Although no significant differences in serum corticosterone concentrations were observed among the experimental groups, paired individuals spent significantly more time resting in the presence of rocks. Our results suggest that underwater rock installations effectively reduce aggression in captive hawksbill turtles.