Summary 1.We apply a cost-benefit model to investigate whether bimodal turtles change their diving behaviour in response to changing energetic costs of aerial vs. aquatic respiration. This question is significant both in the context of the evolution of specialized respiratory structures in bimodal turtles, and the ecological role of respiratory partitioning. 2. Elseya albagula is a bimodally respiring turtle that can extract oxygen from water via specialized cloacal bursae and can vary its reliance on aquatic respiration. We examined the effect of water depth, velocity and temperature on the surfacing frequency of E. albagula through analysis of submergence durations (dive length). We applied a model of resource gain maximization to predict that as the cost of surfacing increases there will be longer dives made possible by the increased reliance on aquatic respiration. Diving behaviour of juvenile turtles was recorded in a large observation tank at varying water depths (50, 100 and 150 cm) and temperatures (20, 25 and 30 ° C). Diving behaviour of adult turtles was also monitored in a flume at water velocities of 5, 15 and 30 cm s -1 . 3. Dive length significantly increased with water depth, with dives at 150 cm twofold longer than dives at 50 cm. Dive length was also significantly influenced by temperature, with shorter dives at 30 ° C than at 20 ° C. In contrast, water velocity had no effect on dive length or on the proportion of a trial adult E. albagula spent using a velocity refuge. 4. Juvenile E. albagula possess the flexibility to adjust dive length in response to changes in water depth and temperature in a manner we would expect if energy use were being optimized. We advance the proposition that the driving force for the evolution of specialized aquatic gas exchange structures in E. albagula is the reduced cost of transport to and from the surface.